Carts: Take creative mode into account

This commit makes the carts mod not add the cart
item to the player's inventory if running with
creative mode and the player already has it.

mods/carts/cart_entity.lua

@@ -91,9 +91,13 @@ function cart_entity:on_punch(puncher, time_from_last_punch, tool_capabilities,
 			end
 		end
 
-		local leftover = puncher:get_inventory():add_item("main", "carts:cart")
-		if not leftover:is_empty() then
-			minetest.add_item(self.object:getpos(), leftover)
+		local inv = puncher:get_inventory()
+		if not minetest.setting_getbool("creative_mode")
+				or not inv:contains_item("main", "carts:cart") then
+			local leftover = inv:add_item("main", "carts:cart")
+			if not leftover:is_empty() then
+				minetest.add_item(self.object:getpos(), leftover)
+			end
 		end
 		self.object:remove()
 		return