small addition to the fix for #3188

dropping of tables added to the testcase

mysql-test/r/subselect.result

@@ -1706,3 +1706,4 @@ create table t2(id int);
 create table t3(flag int);
 select (select * from t3 where id not null) from t1, t2;
 ERROR 42000: You have an error in your SQL syntax.  Check the manual that corresponds to your MySQL server version for the right syntax to use near 'null) from t1, t2' at line 1
+drop table t1,t2,t3;

mysql-test/t/subselect.test

@@ -1114,3 +1114,4 @@ create table t2(id int);
 create table t3(flag int);
 -- error 1064
 select (select * from t3 where id not null) from t1, t2;
+drop table t1,t2,t3;