iscsi-target: Fix NULL pointer dereference in iscsit_send_reject

Fix up a NULL pointer dereference regression in iscsit_send_reject()
introduced by from commit 2ec5a8c1.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>