avcodec/wavpack: Fix runtime error: shift exponent 32 is too large for 32-bit type 'int'

Fixes: 822/clusterfuzz-testcase-4873433189974016

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