Fix H.264 interframe decoding when compiling with icc. Patch by Loren

Merritt:

"It seems that icc copies the constants from their global var onto the
stack, at which point they're not aligned, hence the crash.
[This change] really shouldn't mean anything different, but maybe it'll
confuse icc into not performing that 'optimization'."

Originally committed as revision 12772 to svn://svn.ffmpeg.org/ffmpeg/trunk