staging: most: usb: check for NULL device

Check if the dci structer has been allocated before trying to release it.
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Link: https://lore.kernel.org/r/1579793906-5054-8-git-send-email-christian.gromm@microchip.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/most/usb/usb.c

@@ -1218,7 +1218,8 @@ static void hdm_disconnect(struct usb_interface *interface)
 	del_timer_sync(&mdev->link_stat_timer);
 	cancel_work_sync(&mdev->poll_work_obj);
 
-	device_unregister(&mdev->dci->dev);
+	if (mdev->dci)
+		device_unregister(&mdev->dci->dev);
 	most_deregister_interface(&mdev->iface);
 
 	kfree(mdev->busy_urbs);