a fix (Bug #4102 Crash with a DBUG window after a request)

sql/item.h

@@ -820,7 +820,10 @@ class Item_ref :public Item_ident
   void save_org_in_field(Field *field)	{ (*ref)->save_org_in_field(field); }
   enum Item_result result_type () const { return (*ref)->result_type(); }
   enum_field_types field_type() const   { return (*ref)->field_type(); }
-  table_map used_tables() const		{ return (*ref)->used_tables(); }
+  table_map used_tables() const		
+  { 
+    return depended_from ? OUTER_REF_TABLE_BIT : (*ref)->used_tables(); 
+  }
   void set_result_field(Field *field)	{ result_field= field; }
   bool is_result_field() { return 1; }
   void save_in_result_field(bool no_conversions)