avcodec/wavpack: Fix runtime error: signed integer overflow: 2147483642 + 512 cannot be represented in type 'int'

Fixed: 1453/clusterfuzz-testcase-minimized-5024976874766336

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