Unsorted result for query with ORDER BY if ref_or_null access is used

The following sequence of commands produces unsorted result in MariaDB 5.2 though sorting is asked for:

CREATE TABLE t1 (
  a INT,
  c INT,
  UNIQUE KEY a_c (a,c),
  KEY (a)) engine=myisam;

INSERT INTO t1 VALUES (1,10), (2,NULL), (2,10);
ANALYZE TABLE t1;
EXPLAIN
SELECT c FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;
SELECT c FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;

MariaDB [test]> SELECT c FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;
+------+
| c |
+------+
| 10 |
| NULL |
+------+

In MariaDB the test case works correctly.

See also bug #13531865 from mysql-trunk code line.