The threshold is chosen so that the worse frames would together not take
excessive time.
A better solution is welcome!

Fixes: Timeout (308sec ->102ms)
Fixes: 18314/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VP9_fuzzer-5701689176227840

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