scsi: target: Write NULL to *port_nexus_ptr if no ISID

This patch fixes a minor flaw that could be triggered by a PR OUT RESERVE
on iSCSI, if TRANSPORT IDs with and without ISID are used in the same
command.  In case an ISCSI Transport ID has no ISID, port_nexus_ptr was not
used to write NULL, so value from previous call might persist.  I don't
know if that ever could happen, but with the change the code is cleaner, I
think.

Link: https://lore.kernel.org/r/20200408132610.14623-2-bstroesser@ts.fujitsu.com
Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_fabric_lib.c

@@ -341,7 +341,8 @@ static char *iscsi_parse_pr_out_transport_id(
 			*p = tolower(*p);
 			p++;
 		}
-	}
+	} else
+		*port_nexus_ptr = NULL;
 
 	return &buf[4];
 }