added a test case.

And I am not able to find a Changelog for 4.1

mysql-test/r/union.result

@@ -197,3 +197,5 @@ a
 11
 set SQL_SELECT_LIMIT=DEFAULT;
 drop table t1,t2;
+select * union select 1;
+No tables used

mysql-test/t/union.test

@@ -98,3 +98,6 @@ set SQL_SELECT_LIMIT=2;
 (select * from t1 limit 1) union (select * from t2 limit 3);
 set SQL_SELECT_LIMIT=DEFAULT;
 drop table t1,t2;
+
+--error 1096
+select * union select 1;