target: Fix target_sense_desc_format NULL pointer dereference

This patch allows target_sense_desc_format() to be called without a
valid se_device pointer, which can occur during an early exception
ahead of transport_lookup_cmd_lun() setting up se_cmd->se_device.

This addresses a v4.3-rc1 specific NULL pointer dereference
regression introduced by commit 4e4937e8.

Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_hba.c

@@ -187,5 +187,5 @@ core_delete_hba(struct se_hba *hba)
 
 bool target_sense_desc_format(struct se_device *dev)
 {
-	return dev->transport->get_blocks(dev) > U32_MAX;
+	return (dev) ? dev->transport->get_blocks(dev) > U32_MAX : false;
 }