In jit slowmem, verify actual address.

Oops, it could crash if it was near the boundary.
Well, it still could if it were very near, but that's rare.
Validate jumps in jit slowmem mode.

This makes it easier to see what is going on in the emulator debugger.
Merge pull request #636 from unknownbrackets/jit-minor

Fix slowmem check and add jump check