This is identical to what the VP9 parser does

Fixes: 9215/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_LIBVPX_VP8_fuzzer-5768227253649408
Fixes: out of memory access

This may also fix oss fuzz issue 9212

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>