Fixed logic in Deathball's updateRollAnimation

Previously, there was some odd logic that made the animation speed up incredibly fast, eventually hitting `inf` and crashing the engine. This fixes the script-side issue; perhaps there should be some code-side checks in place to stop this sort of thing happening. (In this case, `setAnimationTimescale` was accepting an infinite value from scripts.)

Anyway, I just wanted to see the deathball speed up as I crushed the poor little soldiers. Now it does. Enjoy.