User controlled cars can drive off the map

[CurryKitten]

Looks like I forgot to do any bounds checking for the user-driveable cars, as a user noted he could drive off the map and fall into an endless void. I have confirmed this, so will add some bounds checking to prevent this.

[CurryKitten]

Fixed - the bounds checking was slightly different from the quad due to the increased size of the car with the same code resulting in the front wheels hanging over the edge of the void :)