avcodec/wavpack: Fix runtime error: signed integer overflow: 2081021665 - -130689706 cannot be represented in type 'int'

Fixes: 2038/clusterfuzz-testcase-minimized-4521466148159488

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