Fix scsi_device_get to allow NULL devices

Modification of patch from SLES-9

drivers/scsi/scsi.c

@@ -977,6 +977,8 @@ int scsi_track_queue_full(struct scsi_device *sdev, int depth)
  */
 int scsi_device_get(struct scsi_device *sdev)
 {
+	if(!sdev)
+		return -ENXIO;
 	if (sdev->sdev_state == SDEV_DEL || sdev->sdev_state == SDEV_CANCEL)
 		return -ENXIO;
 	if (!get_device(&sdev->sdev_gendev))