JIT: fix RAM check in load-from-constant-address

A bug that seems to have been uncovered by allowing immediate-address loads.
Super Monkey Ball 2 crashes without this change -- it's possible, however, that
the game actually requires the MMU hack, since it crashed due to accessing an
address in the 0x20000000-0x3fffffff range.
Merge pull request #899 from FioraAeterna/checkram

JIT: fix RAM check in load-from-constant-address