Fix gravity CVAR crashing when changed from the menu (resolves #106)
Any unbounded floating point cvars placed in the menu will crash, so I also added a failsafe return there.
I could've added handling for unbound cvars but there's really no reason to set the gravity to 999999 or something, it just acts like zero gravity if you set it that high. The game is already pretty broken at the max I set it to.