avcodec/mpeg12dec: Redesign index checks for mpeg2_fast_decode_block_intra

This fixes the speed regression from 20626f53e9f41cb3db82329ed3db7d773cfa3a8f
and still checks sufficiently to prevent out of allocated memory accesses
due to the index

Before:
1681 decicycles in mpeg2_fast_decode_block_intra, 4194238 runs, 66 skips
After:
1658 decicycles in mpeg2_fast_decode_block_intra, 4194248 runs, 56 skips

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
