Memory leak when executing PS for query with IN subquery

CREATE TABLE t1 (a varchar(10)) ENGINE=MYISAM;

CREATE TABLE t2 (b varchar(10) CHARACTER SET utf8) ENGINE=MYISAM;

INSERT INTO t1 VALUES ('b'), ('a'), ('c');

INSERT INTO t2 VALUES ('c'), ('d'), ('b');

PREPARE stmt FROM "SELECT t1.a FROM t1 WHERE t1.a IN (SELECT t2.b FROM t2)";

EXECUTE stmt;

EXECUTE stmt;

DEALLOCATE PREPARE stmt;

DROP TABLE t1,t2;

       Item_direct_ref_to_item *ref=

         new (thd->mem_root) Item_direct_ref_to_item(thd, *arg);