not really change the code (x86_64/gcc-4.3.3 results stay unchanged) but avoids
several casts and an aliasing violation that broke decoding at least on
PPC64/gcc-4.1.2.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@18541 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b