commit-reach: fix memory leak in get_reachable_subset()

This is a leak that has existed since the method was first created
in fcb2c07 (commit-reach: implement get_reachable_subset,
2018-11-02).

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-reach.c

@@ -936,6 +936,8 @@ struct commit_list *get_reachable_subset(struct commit **from, int nr_from,
 		}
 	}
 
+	clear_prio_queue(&queue);
+
 	clear_commit_marks_many(nr_to, to, PARENT1);
 	clear_commit_marks_many(nr_from, from, PARENT2);