Fixes: assertion failure
Fixes: 19301/clusterfuzz-testcase-minimized-ffmpeg_BSF_AV1_FRAME_MERGE_fuzzer-5743212006473728

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