avcodec/wavpack: Check float_shift

Fixes: runtime error: shift exponent 40 is too large for 32-bit type 'unsigned int'
Fixes: 1898/clusterfuzz-testcase-minimized-5970744880136192

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