avcodec/wavpack: Fix invalid shift

Fixes: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'
Fixes: 2377/clusterfuzz-testcase-minimized-6108505935183872

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