Entity: Don't takeDamage on 0 hit points

Entity.cpp

@@ -62,7 +62,10 @@ bool Entity::step(Position const &PlayerPos, FloatSeconds const & Frametime, Lev
 
 int Entity::takeDamage(Color color)
 {
-	hitpoints = std::max((hitpoints - 1), 0);
+	if(hitpoints == 0) {
+		return hitpoints;
+	}
+	hitpoints--;
 	hittime = HIT_TIME;
 	hitcolor = color;
 	return hitpoints;