avcodec/wavpack: Fix runtime error: shift exponent 137 is too large for 32-bit type 'int'

Fixes: 808/clusterfuzz-testcase-4715513349406720

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