avcodec/wavpack: Change wp_log2() to unsigned

Fixes: runtime error: signed integer overflow: 2143315325 + 4186162 cannot be represented in type 'int'
Fixes: 2134/clusterfuzz-testcase-minimized-4619258405322752

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