Fixes: out of array read
Fixes: 15263/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_M101_fuzzer-5728999453491200

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 89b96900)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>