target: Fix ERR_PTR dereferencing bugs

transport_init_session() and core_tmr_alloc_req() never return NULL,
they only return ERR_PTRs on error.

v2: Fix patch to return PTR_ERR(tl_nexus->se_sess) from Ankit Jain's
feedback.

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