libmsc/gsm_09_11.c: do not abuse LOG_TRANS() and early trans allocation

In case of network-originated SS/USSD session establishment, we
need to verify the received GSUP PROC_SS_REQ message and make
sure that all mandatory IEs are present.

There is no sensible need to allocate a new transaction before
doing all the checks, other than the ability to use LOG_TRANS().
This complicates the code, so let's avoid the early allocation.

Change-Id: I4e027b19e8065a39324a1647957cef4066b82ce7
diff --git a/tests/msc_vlr/msc_vlr_test_ss.err b/tests/msc_vlr/msc_vlr_test_ss.err
index 1199cf1..7845c99 100644
--- a/tests/msc_vlr/msc_vlr_test_ss.err
+++ b/tests/msc_vlr/msc_vlr_test_ss.err
@@ -367,9 +367,9 @@
   llist_count(&vsub->cs.requests) == 0
 <-- GSUP rx OSMO_GSUP_MSGT_PROC_SS_REQUEST: 20010809710000004026f03004200001013101013515a11302010102013b300b04010f0406aa510c061b010a0103
 DREF VLR subscr IMSI-901700000004620:MSISDN-46071 + gsm0911_gsup_rx: now used by 3 (attached,_test_ss_ussd_no,gsm0911_gsup_rx)
+DSS (IMSI-901700000004620:MSISDN-46071) Establishing a network-originated session (id=0x20000101)
 DREF VLR subscr IMSI-901700000004620:MSISDN-46071 + NCSS: now used by 4 (attached,_test_ss_ussd_no,gsm0911_gsup_rx,NCSS)
-DSS trans(NCSS IMSI-901700000004620:MSISDN-46071 callref-0x20000101 tid-255) New transaction
-DSS trans(NCSS IMSI-901700000004620:MSISDN-46071 callref-0x20000101 tid-255) Establishing network-originated session
+DSS trans(NCSS IMSI-901700000004620:MSISDN-46071 callref-0x20000101 tid-0) New transaction
 DSS trans(NCSS IMSI-901700000004620:MSISDN-46071 callref-0x20000101 tid-0) Triggering Paging Request
 DPAG Paging: IMSI-901700000004620:MSISDN-46071 for GSM 09.11 SS/USSD: Starting paging
   paging request (SIGNALLING_HIGH_PRIO) to IMSI-901700000004620:MSISDN-46071 on GERAN-A