Fixes: out of array read
Fixes: 20494/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MVDV_fuzzer-5681452423577600
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>