avformat/iff: Check that we have a stream in read_dst_frame()

avformat/iff: Check that we have a stream in read_dst_frame()

Fixes: null pointer dereference
Fixes: 385644864/clusterfuzz-testcase-minimized-ffmpeg_dem_IFF_fuzzer-4551049565765632

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Peter Ross <[email protected]>
Signed-off-by: Michael Niedermayer <[email protected]>
(cherry picked from commit 8668957ef604bd2b99175310638bc5031ae0d991)
Signed-off-by: Michael Niedermayer <[email protected]>