Assertion abort on SELECT @user_var

The following test case causes an assertion abort in MariaDB 5.2:

CREATE TABLE t1(a INT) ENGINE=InnoDB;
INSERT INTO t1 VALUES (0);
SELECT DISTINCT POW(COUNT(*), @a:=(SELECT 1 FROM t1 LEFT JOIN t1 AS t2 ON @a))
AS b FROM t1 GROUP BY a;
SELECT @a;
DROP TABLE t1;
CREATE TABLE t1(f1 INT, f2 INT);
INSERT INTO t1 VALUES (1,2),(2,3),(3,1);
CREATE TABLE t2(a INT);
INSERT INTO t2 VALUES (1);
SET @var=NULL;
SELECT @var:=(SELECT f2 FROM t2 WHERE @var) FROM t1 GROUP BY f1 ORDER BY f2 DESC
LIMIT 1;
SELECT @var;
DROP TABLE t1, t2;

The is no problem here with MaraiDB 5.5 though.