Fixes: out of array access
Fixes: 18788/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DVVIDEO_fuzzer-6254863113781248

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Baptiste Coudurier <baptiste.coudurier@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>