Mark index as unsigend so gcc knows it doesnt need a sign extension on x86_64.

Mark index as unsigend so gcc knows it doesnt need a sign extension on x86_64.
Any tips on how i can convince gcc that it doesnt need a
mov     %eax, %eax
in every get_bits() ?

Originally committed as revision 21433 to svn://svn.ffmpeg.org/ffmpeg/trunk