Fixes: signed integer overflow: 2314885530818453536 - -8926099139098304480 cannot be represented in type 'long'
Fixes: 15259/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5764366093254656

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