avcodec/wavpack: Check shift

Fixes: runtime error: shift exponent 255 is too large for 32-bit type 'unsigned int'
Fixes: 894/clusterfuzz-testcase-4841537823309824

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