iser-target: Fix wrong allocation in the case of an empty text message

if text message dlength is 0, don't allocate a buffer for it, pass
NULL to iscsit_process_text_cmd.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

diff --cc drivers/infiniband/ulp/isert/ib_isert.c
Simple merge