You should. Because absent either the cop being on the property or the dog leaving it, the cop was under no threat. At all. Period.
Your silly fears aside, you don't get to arbitrarily shoot anything on someone else's property with impunity.
Again, your entire argument rests on a charging dog respecting property lines. The law does not require that a charging dog cross property lines before someone can react to the attack.
I do not shoot arbitrarily, but I will shoot when attacked ... by a dog or a man. I don’t care, and the law doesn’t care, if the attack is staged from the attacker’s private property or not.
SnakeDoc