Fixes: division by 0
Fixes: 39562/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AMRWB_fuzzer-5448834960982016
Fixes: 39589/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AMRWB_fuzzer-6119205334810624

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