avcodec/wavpack: Fix multiple runtime error: signed integer overflow: 548 * -2147483648 cannot be represented in type 'int'

Fixes: 1659/clusterfuzz-testcase-minimized-5396490639900672

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