Allow only meshnode glue in controller inventory

If a meshnode controller was trying to make sense of metadata from an
item other than meshnode glue, the server would crash. To prevent this,
only meshnode glue is allowed to be in a meshnode controller inventory.

init.lua

@@ -356,6 +356,9 @@ minetest.register_node("meshnode:controller", {
 		show_meshnode_formspec(pos, clicker)
 	end,
 	allow_metadata_inventory_put = function(pos, listname, index, stack, player)
+		if stack:get_name() ~= "meshnode:glue" then
+			return 0
+		end
 		local meta = stack:get_metadata()
 		if meta then
 			if vector.equals(minetest.string_to_pos(meta), pos) then