gmm: Fix assertion hit during RA UPD REQ before completting gmm attach
Output:
20191107021548500 DMM <0002> gprs_gb.c:40 MM_STATE_Gb(2596296189)[0x6120000084a0]{Idle}: Received Event E_MM_PDU_RECEPTION
20191107021548500 DMM <0002> gprs_gmm.c:1531 MM(/d4b6d7af) -> GMM RA UPDATE REQUEST type="RA updating"
20191107021548501 DMM <0002> gprs_gmm.c:1615 MM(/d4b6d7af) The MM context cannot be used, RA: 901-70-2758-208
Assert failed mmctx->gb.llme == NULL gprs_gmm.c:1620
Scenario reproducing the crash can be triggered with TTCN3
SGSN_Tests.TC_attach_req_id_req_ra_update.
Basically, SGSN first receives an ATTACH REQ with a given RA ID, then
SGSN switches to state CommonProcedureInitiated and sends GMM ID REQ,
and MS/PCU answers immediatelly with a RA Update instead with a new RA
ID.
Related: OS#3957, OS#4245
Change-Id: I64fa5cf1b427d3abb99e553e584897261a827ce6