fix coverity warning CID: 255 (uninitalized variable used to build tables which arent used, well there is a slight change of a FPU exception maybe ...)
(by michael)
Init simplification and 2% faster wma_decode_block on amd64 with tables use
instead of pow(). (by banan)

the init simplifictaion is not correct its not binary identical
the pow optimization breaks decoding of http://klif-e.mos.net.au/videos/extr.wmv

Originally committed as revision 5247 to svn://svn.ffmpeg.org/ffmpeg/trunk