Fixes: 9211/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_GSM_fuzzer-5680396581732352
Fixes: assertion failure

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