LP:1001092 - Query with GROUP BY returns an extra row

The following test case produces a wrong result in MariaDB 5.2:

CREATE TABLE t1 (vc varchar(1), INDEX vc_idx (vc)) ;
INSERT INTO t1 VALUES (NULL), ('o'), (NULL), ('p'), ('c');
FLUSH TABLE t1;
SELECT vc FROM t1 GROUP BY vc;

MariaDB [test]> SELECT vc FROM t1 GROUP BY vc;
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5 rows in set (0.01 sec)

(See also bug #12837714 in mysql-trunk)