avcodec/wavpack: Fix signed integer overflow: 1285114081 * 2 cannot be represented in type 'int'

Fixes: 945/clusterfuzz-testcase-6037937588273152
Fixes: integer overflow

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