When is pr_reg->isid_present_at_reg is false,this function should return.

This fixes a regression originally introduced by:

  commit d2843c17
  Author: Andy Grover <agrover@redhat.com>
  Date:   Thu May 16 10:40:55 2013 -0700

      target: Alter core_pr_dump_initiator_port for ease of use
Signed-off-by: tangwenji <tang.wenji@zte.com.cn>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>