S1AP_Emulation: Fix UeContextReleaseCmd not forwarded to users

We still want to forward it to the user so that it can:
1- validate the IUT sent the message
2- answer with a UeContextReleaseComplete

The f_assoc_id_by_s1ap_ids() function is modified to avoir a Dynamic
test error if the enb_ue_id is omit, which can happen if
UeContextRleaseCmd used choice "MME UE S1AP ID".

Change-Id: Iaf6a775d1cbc4f85f185bb26a0da1e5c9a4167a2
1 file changed