tcm_fc: move the dereference below the NULL test

The dereference should be moved below the NULL test.

spatch with a semantic match is used to found this.
(http://coccinelle.lip6.fr/)
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NNicholas Bellinger <nab@linux-iscsi.org>

drivers/target/tcm_fc/tfc_io.c

@@ -327,11 +327,12 @@ void ft_recv_write_data(struct ft_cmd *cmd, struct fc_frame *fp)
  */
 void ft_invl_hw_context(struct ft_cmd *cmd)
 {
-	struct fc_seq *seq = cmd->seq;
+	struct fc_seq *seq;
 	struct fc_exch *ep = NULL;
 	struct fc_lport *lport = NULL;
 
 	BUG_ON(!cmd);
+	seq = cmd->seq;
 
 	/* Cleanup the DDP context in HW if DDP was setup */
 	if (cmd->was_ddp_setup && seq) {