fix server crash

https://github.com/APercy/airutils/issues/10

airutils_tug.lua

@@ -8,6 +8,7 @@ function airutils.move_target(player, pointed_thing)
     --minetest.chat_send_all(dump(object))
     if object then
         local obj_pos = object:get_pos()
+	if not obj_pos then return end
         local hip = math.sqrt(math.pow(obj_pos.x - pos.x,2)+math.pow(obj_pos.z - pos.z,2)) + 1
         local pos_x = math.sin(yaw) * -hip
         local pos_z = math.cos(yaw) * hip