This fixes the speed regression from 20626f53
and still checks sufficiently to prevent out of allocated memory accesses
due to the index

Before:
1823 decicycles in mpeg2_fast_decode_block_non_intra, 8388493 runs, 115 skips
After:
1808 decicycles in mpeg2_fast_decode_block_non_intra, 8388494 runs, 114 skips
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>