avformat/wtvdec: clear sectors

avformat/wtvdec: clear sectors

The code can leave uninitialized holes in the array.
Fixes: use of uninitialized values
Fixes: 70883/clusterfuzz-testcase-minimized-ffmpeg_dem_WTV_fuzzer-6698694567591936

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 c95ea0310468e0a0906fa7d590ff7406c39d6991)
Signed-off-by: Michael Niedermayer <[email protected]>