LP:993745 - Crash when using min/max loose scan

The following sequence of commands causes a crash in mariadb-5.2:

CREATE TABLE t1(a BLOB, b VARCHAR(255) CHARSET LATIN1, c INT, KEY(b, c, a(765))) ENGINE=INNODB;
INSERT INTO t1(a, b, c) VALUES ('', 'a', 0), ('', 'a', null), ('', 'a', 0);
SELECT MIN(c) FROM t1 GROUP BY b;

(see also bug #13723054 for mysql code line)