Fixes: signed integer overflow: 6494225984479297536 - -6043795377581187040 cannot be represented in type 'long'
Fixes: 15285/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5632780307791872

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