Fixes: out of array access
Fixes: 23327/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-5134822992510976

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 e53235f06c229a23d3241b47e32647019161fb7c)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>