Stop undefined backgrounds from freezing game

Addresses issue #27. Apparently the tile and attrib arrays are 32x32, which is bigger than "real" screens (real screens are only 30x32. This causes line 1010 to return an undefined tile when it hits the first byte of row #30 (which isn't initialized properly).

I may be making some assumptions here. Hopefully someone with better knowledge of the system can provide some more insight.