iscsi-target: Fix potential NULL pointer in solicited NOPOUT reject

commit 28aaa950320fc7b8df3f6d2d34fa7833391a9b72 upstream.

This patch addresses a potential NULL pointer dereference regression in
iscsit_setup_nop_out() code, specifically for two cases when a solicited
NOPOUT triggers a ISCSI_REASON_PROTOCOL_ERROR reject to be generated.

This is because iscsi_cmd is expected to be NULL for solicited NOPOUT
case before iscsit_process_nop_out() locates the descriptor via TTT
using iscsit_find_cmd_from_ttt().

This regression was originally introduced in:

commit ba159914086f06532079fc15141f46ffe7e04a41
Author: Nicholas Bellinger <nab@linux-iscsi.org>
Date:   Wed Jul 3 03:48:24 2013 -0700

    iscsi-target: Fix iscsit_add_reject* usage for iser

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>