Fix following coccicheck error:

./drivers/target/iscsi/iscsi_target_nego.c:831:13-26: ERROR: se_nacl is NULL but dereferenced.

Link: https://lore.kernel.org/r/20220613014321.107045-1-yang.lee@linux.alibaba.com
Fixes: a75fcb09 ("scsi: target: iscsi: Extract auth functions")
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>