After making remoteport unregister requests, the ndlp nrport pointer was
stale.

Track when waiting for waiting for unregister completion callback and
adjust nldp pointer assignment.  Add a few safety checks for NULL
pointer values.
Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>