iscsi-target: Fix NULL dereference on allocation failure

This patch fixes a bug in iscsi_target_init_negotiation() where
the "goto out" path dereferences "login" which is NULL upon a
memory allocation failure.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>