LP:825075 - Wrong result with multipart key + MIN/MAX loose scan and OR expression in WHERE

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 ( a int, b varchar(1), KEY (b,a));

INSERT INTO t1 VALUES (0,NULL),(9,NULL),(8,'c'),(4,'d'),(7,'d'),(NULL,'f'),(7,'f'),(8,'g'),(NULL,'j');

SELECT a , b FROM t1 WHERE a IS NULL OR b = 'z' ;

SELECT max(a) , b FROM t1 WHERE a IS NULL OR b = 'z' GROUP BY b;