Clippers don't really work. Previously we mostly didn't run into this because
the rectangle was already rejected by ddraw_surface7_Blt(), although
ddraw_surface7_BltFast() might have been affected in a couple of cases. We
should of course implement clippers, but until that happens, completely
rejecting the blit is better than introducing memory corruption.

This fixes a regression introduced by commit
92e616f3.