Emulator crash when exiting after address error

[Sik]

If an address error exception happens in the emulated program, BlastEm is prone to crash with a segfault when exiting (doesn't happen every time, it seems more likely to crash if you ever bring up the UI or the debugger, would need to pay more attention to that).

I haven't checked to see if loading another ROM also crashes, if that's the case then priority should be probably bumped to major (though address error is not common unless you're developing homebrew or working on a hack).

Tracker still only goes up to 0.6.2 but the bug is present in 0.6.3.