target/pscsi: Fix NULL pointer dereference in get_device_type

This patch fixes a NULL pointer dereference OOPs with pSCSI backends
within target_core_stat.c code.  The bug is caused by a configfs attr
read if no pscsi_dev_virt->pdv_sd has been configured.

Reported-by: Olaf Hering <olaf@aepfle.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_pscsi.c

@@ -1121,7 +1121,7 @@ static u32 pscsi_get_device_type(struct se_device *dev)
 	struct pscsi_dev_virt *pdv = PSCSI_DEV(dev);
 	struct scsi_device *sd = pdv->pdv_sd;
 
-	return sd->type;
+	return (sd) ? sd->type : TYPE_NO_LUN;
 }
 
 static sector_t pscsi_get_blocks(struct se_device *dev)