Two considerations here -
(a) the potential for overpressure is addressed by the relief valves and these can be backed up by the pilot's attempts to dump pressure manually if it came to that
(b) cabin altitude is a function of air pressure. Whatever pressure exists relates to the same pressure's being scheduled at a particular Hp in the standard atmosphere ie if the pressure in the cabin is the same as, say, that which occurs at 8000ft in the standard atmosphere, then the cabin altitude is 8000ft.
Inverse relationship - ie as the pressure goes down the cabin altitude goes up .. same as in the real world. So, for the case of a defective outflow valve, the pressure might increase a small amount which would see the cabin altitude decrease by the appropriate amount.