Bugfix: only arena_lib signs can't be broken

signs.lua

@@ -10,7 +10,12 @@ local function arena_display_format(arena, msg) end
 
 minetest.override_item("default:sign_wall", {
 
-    on_dig = function() end,
+    on_dig = function(pos, node, digger)
+      if minetest.get_meta(pos):get_int("arenaID") ~= 0 then
+        return end
+
+      minetest.node_dig(pos,node,digger)
+    end,
 
     on_punch = function(pos, node, puncher, pointed_thing)