[5.5] Error: Freeing overrun buffer upon SELECTs with GROUP BY and ROLLUP

CREATE TABLE t1 (i INT) ENGINE=MyISAM;

INSERT INTO t1 VALUES (2),(7),(2),(2),(4),(6),(6),(8),(4),(2),(8),(1),(NULL),(3),(2);

SELECT ( SELECT DISTINCT i FROM `t1` WHERE i GROUP BY @a := 1 WITH ROLLUP ) AS s1;

SELECT NULL FROM `t1` GROUP BY 'binary', LAST_DAY('2012-12-12') WITH ROLLUP;

Error: Freeing overrun buffer  mysys/safemalloc.c:179, mysys/my_malloc.c:120, sql/filesort.cc:320, sql/sql_select.cc:19937, sql/sql_select.cc:2837, sql/sql_select.cc:3096, sql/sql_select.cc:319, sql/sql_parse.cc:4686

Allocated at sql/filesort.cc:219, sql/sql_select.cc:19937, sql/sql_select.cc:1888, sql/sql_select.cc:2402, sql/item_subselect.cc:3269, sql/item_subselect.cc:663, sql/item_subselect.cc:1244, sql/item.cc:6642