avcodec/wavpack: Fix integer overflows in wv_unpack_stereo / mono

Fixes: runtime error: signed integer overflow: 2146276249 + 1487583 cannot be represented in type 'int'
Fixes: 4823/clusterfuzz-testcase-minimized-4551896611160064

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