Fix crash when furnace is full

mods/default/init.lua

@@ -1403,7 +1403,7 @@ minetest.register_abm({
 					srcstack:take_item()
 					inv:set_stack("src", 1, srcstack)
 				else
-					print("Could not insert '"..cooked.item.."'")
+					print("Could not insert '"..cooked.item:to_string().."'")
 				end
 				meta:set_string("src_time", 0)
 			end