dm delay: fix a crash when invalid device is specified

When the target line contains an invalid device, delay_ctr() will call
delay_dtr() with NULL workqueue.  Attempting to destroy the NULL
workqueue causes a crash.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-delay.c

@@ -121,6 +121,7 @@ static void delay_dtr(struct dm_target *ti)
 {
 	struct delay_c *dc = ti->private;
 
+	if (dc->kdelayd_wq)
 		destroy_workqueue(dc->kdelayd_wq);
 
 	if (dc->read.dev)