Fix out-of-bounds memory accesses

[DavidBuchanan314]

If I ends up greater than 0xfff, then the memory array can be indexed out-of-bounds (e.g. repeated use of the ADD I, Vx instruction). This is a security vulnerability. A malicious ROM can leverage this to escape the emulator, and execute arbitrary native code on the host system.

I noticed this comment in cpu.h:

chip8/src/lib8/cpu.h

Lines 49 to 54 in d952e57

	/**
	* This is the maximum amount of memory addressable by the machine.
	* It should be used as a bitmask when overflows could possibly happen
	* due to memory address management.
	*/
	#define ADDRESS_MASK 0xFFF

However, this ADDRESS_MASK constant is never used anywhere! So, I used it to add masking to all memory reads and writes, fixing the vulnerability.

[danirod]

That is true, good caught, thanks.