Fixes: division by zero
Fixes: 15725/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5641231956180992

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>