check item exists for hairballs

kitten.lua

@@ -155,7 +155,8 @@ minetest.register_craftitem(":mobs:hairball", {
 		local newpos = {x = pos.x + dir.x, y = pos.y + dir.y + 1.5, z = pos.z + dir.z}
 		local item = hairball_items[math.random(1, #hairball_items)]
 
-		if item ~= "" then
+		if item ~= ""
+		and minetest.registered_items[item] then
 			minetest.add_item(newpos, {name = item})
 		end