avcodec/wavpack: Check value before shift in wp_exp2()

Fixes undefined shift, all callers should be changed to check the value
they use with wp_exp2() or its return value.

Fixes: 692/clusterfuzz-testcase-5757381516460032

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