avcodec/wavpack: Fix integer overflow

Fixes: runtime error: signed integer overflow: 227511904 + 1964113935 cannot be represented in type 'int'
Fixes: 2331/clusterfuzz-testcase-minimized-6182185830711296

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