iscsi-target: Fix panic when adding second TCP connection to iSCSI session

In MC/S scenario, the conn->sess has been set NULL in
iscsi_login_non_zero_tsih_s1 when the second connection comes here,
then kernel panic.

The conn->sess will be assigned in iscsi_login_non_zero_tsih_s2. So
we should check whether it's NULL before calling.
Signed-off-by: Feng Li <lifeng1519@gmail.com>
Tested-by: Sumit Rai <sumit.rai@calsoftinc.com>
Cc: stable@vger.kernel.org # 3.14+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_login.c

@@ -1371,8 +1371,9 @@ static int __iscsi_target_login_thread(struct iscsi_np *np)
 	}
 	login->zero_tsih = zero_tsih;
 
-	conn->sess->se_sess->sup_prot_ops =
-		conn->conn_transport->iscsit_get_sup_prot_ops(conn);
+	if (conn->sess)
+		conn->sess->se_sess->sup_prot_ops =
+			conn->conn_transport->iscsit_get_sup_prot_ops(conn);
 
 	tpg = conn->tpg;
 	if (!tpg) {