Fixed bug mdev-5091.

The function SELECT_LEX::update_used_tables should process all
ORDER BY lists in any subselect of a union.

mysql-test/r/union.result

@@ -1724,3 +1724,18 @@ foo
 1
 deallocate prepare stmt1;
 End of 5.1 tests
+#
+# mdev-5091: Asseirtion failure for UNION with ORDER BY 
+#            in one of selects
+#
+CREATE TABLE t1 (i int, c char(1)) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (6,'b');
+CREATE VIEW v1 AS SELECT * FROM t1;
+( SELECT i FROM v1 GROUP BY i ORDER BY CONCAT( c, c ) LIMIT 1 )
+UNION 
+( SELECT i FROM t1 );
+i
+6
+DROP VIEW v1;
+DROP TABLE t1;
+End of 5.3 tests