LP:1001506 - Crash on a query with GROUP BY and user variables

The following simple test case causes a crash of the server in MariaDB 5.2/5.3/5.5:

CREATE TABLE t1(a int);
INSERT INTO t1 VALUES (1), (2);
SELECT DISTINCT @a:=MIN(t1.a) FROM t1, t1 AS t2
GROUP BY @b:=(SELECT COUNT > t2.a);
DROP TABLE t1;

(See also bug #57197 from bugs.mysql.com)