this patch fixes a use of uninitialized value discovered by one of the fuzzers
of the oss-fuzz project:
https://github.com/google/oss-fuzz/blob/master/projects/zlib/example_dict_fuzzer.c
clang's memory sanitizer fails with:

==1==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x5930dd in slide_hash zlib/deflate.c:222:20
    madler#1 0x589461 in fill_window zlib/deflate.c:1558:13
    madler#2 0x59828f in deflate_rle zlib/deflate.c:2119:13
    madler#3 0x590614 in deflate zlib/deflate.c:1045:41
    madler#4 0x4a2d56 in test_dict_deflate /src/example_dict_fuzzer.c:79:11
    madler#5 0x4a3e9b in LLVMFuzzerTestOneInput /src/example_dict_fuzzer.c:156:3
    madler#6 0x4ed04b in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:571:15
    madler#7 0x4a4ff6 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/libfuzzer/FuzzerDriver.cpp:280:6
    madler#8 0x4b5e1a in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:713:9
    madler#9 0x4a4121 in main /src/libfuzzer/FuzzerMain.cpp:20:10
    madler#10 0x7f3d7a13b82f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291
    madler#11 0x41ed08 in _start
  Uninitialized value was created by a heap allocation
    #0 0x45fa10 in malloc /src/llvm/projects/compiler-rt/lib/msan/msan_interceptors.cc:911
    madler#1 0x586920 in deflateInit2_ zlib/deflate.c:320:27
    madler#2 0x4a2a24 in test_dict_deflate /src/example_dict_fuzzer.c:61:11
    madler#3 0x4a3e9b in LLVMFuzzerTestOneInput /src/example_dict_fuzzer.c:156:3
    madler#4 0x4ed04b in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:571:15
    madler#5 0x4a4ff6 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/libfuzzer/FuzzerDriver.cpp:280:6
    madler#6 0x4b5e1a in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:713:9
    madler#7 0x4a4121 in main /src/libfuzzer/FuzzerMain.cpp:20:10
    madler#8 0x7f3d7a13b82f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291

…eflate_quick

by aggregating the two consecutive values to be written by static_emit_ptr to
s->pending_buf and writing the two values at once in a 4 byte store, we avoid
running out of the allocated buffer. We used to call quick_send_bits twice and
bumped the counter s->pending in the first call, which made the second call
write to memory beyond the safe 4 bytes that were guaranteed by the following
condition in the enclosing loop in deflate_quick:

  if (s->pending + 4 >= s->pending_buf_size) {
    flush_pending(s->strm);

The bug was exposed by the memory sanitizer like so:

MemorySanitizer:DEADLYSIGNAL
--
  | ==1==ERROR: MemorySanitizer: SEGV on unknown address 0x730000020000 (pc 0x0000005b6ce4 bp 0x7fff59adb5e0 sp 0x7fff59adb570 T1)
  | ==1==The signal is caused by a WRITE memory access.
  | #0 0x5b6ce3 in quick_send_bits zlib-ng/arch/x86/deflate_quick.c:134:48
  | madler#1 0x5b5752 in deflate_quick zlib-ng/arch/x86/deflate_quick.c:243:21
  | madler#2 0x590a15 in zng_deflate zlib-ng/deflate.c:952:18
  | madler#3 0x587165 in zng_compress2 zlib-ng/compress.c:59:15
  | madler#4 0x5866d3 in check_compress_level zlib-ng/test/fuzz/compress_fuzzer.c:22:3
  | madler#5 0x5862d8 in LLVMFuzzerTestOneInput zlib-ng/test/fuzz/compress_fuzzer.c:74:3
  | madler#6 0x4e9b48 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:575:15
  | madler#7 0x4a2f66 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/libfuzzer/FuzzerDriver.cpp:280:6
  | madler#8 0x4b3adb in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:715:9
  | madler#9 0x4a2091 in main /src/libfuzzer/FuzzerMain.cpp:20:10
  | madler#10 0x7fb8919b082f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291
  | madler#11 0x41ec68 in _start
  | MemorySanitizer can not provide additional info.
  | SUMMARY: MemorySanitizer: SEGV (/mnt/scratch0/clusterfuzz/slave-bot/builds/clusterfuzz-builds_zlib-ng_7ead0a3e4980f024583384fd355b6e3ddd4b2ca2/revisions/compress_fuzzer+0x5b6ce3)

…ed-value

==1==WARNING: MemorySanitizer: use-of-uninitialized-value
  #0 0x59fa93 in deflate_medium zlib-ng/deflate_medium.c:259:21
  madler#1 0x590905 in zng_deflate zlib-ng/deflate.c:951:18
  madler#2 0x587095 in zng_compress2 zlib-ng/compress.c:59:15
  madler#3 0x5866e3 in check_compress_level zlib-ng/test/fuzz/compress_fuzzer.c:18:3
  madler#4 0x5862fd in LLVMFuzzerTestOneInput zlib-ng/test/fuzz/compress_fuzzer.c:38:3
  madler#5 0x4e9b48 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:575:15
  madler#6 0x4a2f66 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/libfuzzer/FuzzerDriver.cpp:280:6
  madler#7 0x4b3adb in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:715:9
  madler#8 0x4a2091 in main /src/libfuzzer/FuzzerMain.cpp:20:10
  madler#9 0x7fea2fea482f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291
  madler#10 0x41ec68 in _start
Uninitialized value was created by a heap allocation
  #0 0x45f2a0 in malloc /src/llvm/projects/compiler-rt/lib/msan/msan_interceptors.cc:910
  madler#1 0x587d42 in zng_deflateInit2_ zlib-ng/deflate.c:284:27
  madler#2 0x5874fa in zng_deflateInit_ zlib-ng/deflate.c:224:12
  madler#3 0x586c95 in zng_compress2 zlib-ng/compress.c:41:11
  madler#4 0x5866e3 in check_compress_level zlib-ng/test/fuzz/compress_fuzzer.c:18:3
  madler#5 0x5862fd in LLVMFuzzerTestOneInput zlib-ng/test/fuzz/compress_fuzzer.c:38:3
  madler#6 0x4e9b48 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:575:15
  madler#7 0x4a2f66 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/libfuzzer/FuzzerDriver.cpp:280:6
  madler#8 0x4b3adb in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:715:9
  madler#9 0x4a2091 in main /src/libfuzzer/FuzzerMain.cpp:20:10
  madler#10 0x7fea2fea482f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291

==4908==ERROR: MemorySanitizer: SEGV on unknown address 0x730fffffffff (pc 0x0000004b1b97 bp 0x7ffd4bf59a00 sp 0x7ffd4bf598a0 T4908)
==4908==The signal is caused by a READ memory access.
  #0 0x5a0599 in fizzle_matches zlib-ng/deflate_medium.c:168:12
  madler#1 0x59ea27 in deflate_medium zlib-ng/deflate_medium.c:296:21
  madler#2 0x5901c5 in zng_deflate zlib-ng/deflate.c:951:18
  madler#3 0x586955 in zng_compress2 zlib-ng/compress.c:59:15
  madler#4 0x5861eb in LLVMFuzzerTestOneInput zlib-ng/test/fuzz/compress_fuzzer.c:18:3
  madler#5 0x4e9b48 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:575:15
  madler#6 0x4a2f66 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/libfuzzer/FuzzerDriver.cpp:280:6
  madler#7 0x4b3adb in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:715:9
  madler#8 0x4a2091 in main /src/libfuzzer/FuzzerMain.cpp:20:10
  madler#9 0x7fa3d7ff582f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291
  madler#10 0x41ec68 in _start

* faster crc32 of last <8 bytes on aarch64

this patch fixes a use of uninitialized value discovered by one of the fuzzers
of the oss-fuzz project:
https://github.com/google/oss-fuzz/blob/master/projects/zlib/example_dict_fuzzer.c
clang's memory sanitizer fails with:

==1==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x5930dd in slide_hash zlib/deflate.c:222:20
    madler#1 0x589461 in fill_window zlib/deflate.c:1558:13
    madler#2 0x59828f in deflate_rle zlib/deflate.c:2119:13
    madler#3 0x590614 in deflate zlib/deflate.c:1045:41
    madler#4 0x4a2d56 in test_dict_deflate /src/example_dict_fuzzer.c:79:11
    madler#5 0x4a3e9b in LLVMFuzzerTestOneInput /src/example_dict_fuzzer.c:156:3
    madler#6 0x4ed04b in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:571:15
    madler#7 0x4a4ff6 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/libfuzzer/FuzzerDriver.cpp:280:6
    madler#8 0x4b5e1a in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:713:9
    madler#9 0x4a4121 in main /src/libfuzzer/FuzzerMain.cpp:20:10
    madler#10 0x7f3d7a13b82f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291
    madler#11 0x41ed08 in _start
  Uninitialized value was created by a heap allocation
    #0 0x45fa10 in malloc /src/llvm/projects/compiler-rt/lib/msan/msan_interceptors.cc:911
    madler#1 0x586920 in deflateInit2_ zlib/deflate.c:320:27
    madler#2 0x4a2a24 in test_dict_deflate /src/example_dict_fuzzer.c:61:11
    madler#3 0x4a3e9b in LLVMFuzzerTestOneInput /src/example_dict_fuzzer.c:156:3
    madler#4 0x4ed04b in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:571:15
    madler#5 0x4a4ff6 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/libfuzzer/FuzzerDriver.cpp:280:6
    madler#6 0x4b5e1a in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:713:9
    madler#7 0x4a4121 in main /src/libfuzzer/FuzzerMain.cpp:20:10
    madler#8 0x7f3d7a13b82f in __libc_start_main /build/glibc-Cl5G7W/glibc-2.23/csu/libc-start.c:291

When there are no symbols in the tree we skip build_tree calculations and emit a block using static tree with no codes.

trees.c:357:19: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'unsigned long'
    #0 0x1000ed79b in build_tree trees.c:357
    madler#1 0x1000ea3f5 in zng_tr_flush_block trees.c:649
    madler#2 0x100090ab0 in deflate_slow deflate_slow.c:131
    madler#3 0x1000572bc in zng_deflate deflate.c:990
    madler#4 0x1000aecd3 in gz_comp gzwrite.c:125
    madler#5 0x1000b05df in zng_gzclose_w gzwrite.c:511
    madler#6 0x1000967a4 in zng_gzclose gzlib.c:253
    madler#7 0x100004f70 in test_gzio example.c:133
    madler#8 0x100010c5b in main example.c:1034
    madler#9 0x7fff71f57cc8 in start+0x0 (libdyld.dylib:x86_64+0x1acc8)

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