Crash when rebooting emulator core

The following Lua script crashes EmuHawk when rebooting the core:

event.onexit(function()
	memory.read_u8(0)
end)

while true do
	emu.frameadvance()
end

This happens because the script is not stopped until LuaConsole.Restart, at which point the old core instance has been disposed. (Of course, even if the exit handler read from the new core, that would not be correct either.)

Repro

1. Open a ROM with a core that supports memory API
2. Start the above Lua script
3. Reload the ROM

Output

No error message.

Host env.

BizHawk 2.11 and current master

Start with the LuaConsole.Restart path and the event.onexit and memory API behavior described in the report. Review pull request #4638 for the work already underway, then reproduce the script while reloading the ROM. Done means the emulator does not crash and reports the failure appropriately when the exit handler accesses the disposed core.