avcodec/wavpack: Fix runtime error: signed integer overflow: -1386217472 * 4 cannot be represented in type 'int'

Fixes: 1853/clusterfuzz-testcase-minimized-5471155626442752

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