Bug #7498 User variable SET saves SIGNED BIGINT as UNSIGNED BIGINT

- Add unsigned flag to user_var_entry, used when 'type' is INT_RESULT
- Propagate unsigned flag from the query executed by Item_single_row_subselect

sql/sql_class.h

@@ -1954,6 +1954,7 @@ class user_var_entry
   ulong length;
   query_id_t update_query_id, used_query_id;
   Item_result type;
+  bool unsigned_flag;
 
   double val_real(my_bool *null_value);
   longlong val_int(my_bool *null_value);