Fixes: out of array read
Fixes: 19300/clusterfuzz-testcase-minimized-ffmpeg_BSF_VP9_METADATA_fuzzer-5653911730126848

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