Fix segfault caused by Toroko's sprite

Some of the bounds checking code was removed from SDL_UpperBlit() by
commit 40126b8. Some frames of Toroko's sprite have a negative y-offset,
so they'll cause a segmentation fault at the start of the Sand Zone
warehouse boss fight if not clipped properly. This commit fixes the
segfault by restoring the old bounds checking code.
Merge pull request #18 from chrisacheson/fix_toroko_sprite_segfault

Fix segfault caused by Toroko's sprite