Fixes: division by zero
Fixes: 15257/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_BINTEXT_fuzzer-5757352881422336

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