Fixes: signed integer overflow: 6175076100092079360 - -5034989061050195840 cannot be represented in type 'long'
Fixes: 18614/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FFWAVESYNTH_fuzzer-5704508847423488

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