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>