ffwavesynth can produce large amounts of data relatively slowly on very small input

Fixes: Timeout (60sec -> 9sec)
Fixes: 17970/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5689121279836160

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