avcodec/wavpack: Fix integer overflow in wv_unpack_stereo()

Fixes: runtime error: signed integer overflow: 2080374785 + 2080374784 cannot be represented in type 'int'
Fixes: 2351/clusterfuzz-testcase-minimized-5359403240783872

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