Fix Lavafalls crashing the game when scaled down.

A modulus operation is used to decide the different rate that larger lavafalls spew lava at. This can cause a divide by zero. The math before did not account for this scenario though, and made it worse by shifting it down, making any value under a FRACUNIT become 0, now it caps off at 1.

Smaller lavafalls act the same as their regular size, might be worth changing that later. (also it'd be really sick if a udmf map scaled one of these up to have it turn on and off constantly)