Bug #51524 valgrind errors with explain, join, unionless merge table

Description:
might be related to bug #51494 but no crashed happened
valgrind 5.1.44 errors:

Conditional jump or move depends on uninitialised value(s)
at: my_wildcmp_bin (ctype-bin.c:337)
by: Item_func_like::val_int (item_cmpfunc.cc:4546)
by: JOIN::optimize (sql_select.cc:6160)
by: mysql_select (sql_select.cc:2445)
by: mysql_explain_union (sql_select.cc:16617)
by: execute_sqlcom_select (sql_parse.cc:5030)
by: mysql_execute_command (sql_parse.cc:2248)
by: mysql_parse (sql_parse.cc:5971)
by: dispatch_command (sql_parse.cc:1233)
by: do_command (sql_parse.cc:874)
by: handle_one_connection (sql_connect.cc:1127)
by: start_thread (in /lib/libpthread-2.5.so)
by: clone (in /lib/libc-2.5.so)

strangely 5.1.45 didn't show error, but 5.1.44 and latest 5.5.99-m3 did:
revno: 2987
committer: Vladislav Vaintroub <vvaintroub@mysql.com>
branch nick: mysql-next-mr-bugfixing
timestamp: Thu 2010-02-25 10:57:23 +0100

How to repeat:
#run mysqld under valgrind;

drop table if exists `t1`;
create table `t1` (`a` int unsigned not null) engine=myisam;
insert into t1 values (1);

drop table if exists `t2`;
create table `t2`(`b` varchar(9) not null, key  (`b`)
) engine=mrg_myisam default charset=latin1;

explain select 1 from t1 natural right outer join t2
where b like 1;

I do not see this specific valgrind error with recent 5.1.45-debug from bzr running under valgrind 3.5.0 on SuSE 9.3.

Duplicate of bug #51494 it seems.