correct fix for MySQL Bug # 19699237: UNINITIALIZED VARIABLE IN ITEM_FIELD::STR_RESULT

create table t1 (a varchar(10) character set utf8);

create table t2 (a varchar(10) character set latin1);

insert t1 values ('aaa');

insert t2 values ('aaa');

select (select concat(a),1 from t1) <=> (select concat(a),1 from t2);

drop table t1, t2;