unitialized handler::key_compare_result_on_equal in loose index scan of partition tables

--source include/have_partition.inc

--source include/have_sequence.inc

CREATE TABLE t1 (

 a INT,

 b INT,

 KEY ( a, b )

) PARTITION BY RANGE COLUMNS(a) (

 PARTITION `p1` VALUES LESS THAN (10),

 PARTITION `p2` VALUES LESS THAN (20),

 PARTITION `p3` VALUES LESS THAN (30),

 PARTITION `p4` VALUES LESS THAN (40),

 PARTITION `p5` VALUES LESS THAN MAXVALUE

);

insert into t1 select seq, seq from seq_1_to_50;

FLUSH status;

SELECT a, MIN(b) FROM t1 WHERE a > 18 and a < 21 GROUP BY a;

show status like 'handler_read_%';

DROP TABLE t1;

int handler::compare_key(key_range *range)

{

  int cmp;

  if (!range || in_range_check_pushed_down)

    return 0;					// No max range

  cmp= key_cmp(range_key_part, range->key, range->length);

  if (!cmp)

    cmp= key_compare_result_on_equal; // <-- breakpoint

  return cmp;

}