LP:693935 - Assert failure for a query with a single row subquery

The following query triggers an assert failure in the current mariadb 5.1 tree:

select * from t2 where a=(select a from t1) and a='x';

To reproduce the failure create and populate t1,t2 with these commands:
create table t1 (a char(1) charset utf8);
insert into t1 values ('a'), ('b');
create table t2 (a binary(1));
insert into t2 values ('x'), ('y');

The failure looks like this (e.g in gdb):
Version: '5.1.54-MariaDB-debug' socket: '/tmp/mysql.sock' port: 3307 Source distribution
[New Thread 0x8ce62b90 (LWP 15193)]
mysqld: item_strfunc.cc:2755: virtual String* Item_func_conv_charset::val_str(String*): Assertion `fixed == 1' failed.

Program received signal SIGABRT, Aborted.