prevent vehicle turrets from attacking their own car

Vehicle turrets use a fake npc to fire their gun. This PR adds checks for that fake creature and the vehicle they are on. It prevents calling `map::shoot` if this would damage the very same vehicle.

This prevents certain code that damages the terrain/furniture from being executed, but the squares should not have any of theses terrain/furnitures, because driving the car onto that square should have already damaged the content.