Fix slowmem check and add jump check

This makes slowmem a little slower, because it validates jr and jalr jumps to ensure they won't go into outer space.

It also checks the offset when validating the memory read/write pointer.  Final Fantasy 4 is currently trying to read from scratchpad + -offset, which was just crashing because that's actually below scratchpad.  This way, it doesn't crash and having "Ignore illegal reads/writes" off works too.

-[Unknown]