avoid crash on searching invalid nodes

rules.lua

@@ -37,16 +37,18 @@ function check_position(pos)
     end
 
     local ret = _check_position_uncached(pos)
-    print(ret) -- todo: remove this
     cache[hash] = ret
     return ret
 end
 
 function _check_position_uncached(pos)
     local node = minetest.get_node(pos)
-
     local reg_item = minetest.registered_items[node.name]
 
+    if not reg_item then
+        return "W"
+    end
+
     if reg_item.groups and reg_item.groups.go_stone then
         return "S" .. reg_item.go_team
     end