Wrong result when using SQL_BUFFER_RESULT if min/max loose scan is chosen

The following sequence of commands brings us to a wrong result in MariaDB 5.2 :

CREATE TABLE t1 (a int, b varchar(1), KEY (b,a) );
INSERT INTO t1 VALUES (1,NULL),(0,'a');
SINSERT INTO t1 VALUES (2,'x'),(0,'y');
SELECT SQL_BUFFER_RESULT MIN(a), b FROM t1 WHERE t1.b = 'a' GROUP BY b;

MariaDB [test]> SELECT SQL_BUFFER_RESULT MIN(a), b FROM t1 WHERE t1.b = 'a' GROUP BY b;
+--------+------+
| MIN(a) | b |
+--------+------+
| NULL | NULL |
+--------+------+

The result is correct without the modifier SQL_BUFFER_RESULT:

MariaDB [test]> SELECT MIN(a), b FROM t1 WHERE t1.b = 'a' GROUP BY b;
+--------+------+
| MIN(a) | b |
+--------+------+
| 0 | a |
+--------+------+

(See also bug #12640437 for mysql-5.6)