Changes

library/NGAP_Functions: return NAS_PDU IE from InitialContextSetup if present

This is required to obtain the NAS PDU transmitted by open5gs-smfd
during Network Triggered Service Request.
In that message, the NAS_PDU inside the
PDUSessionResourceSetupListCxtReq item is empty, and instead the global
NAS_PDU is filled with a Service Accept.

Change-Id: I7fced3f920c2514f58711a2ca1cc60c8dc79e395