avcodec/wavpack: Fix: runtime error: signed integer overflow: 3 * -2147483648 cannot be represented in type 'int'

Fixes: 1776/clusterfuzz-testcase-minimized-6191258231898112

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