Allocate and add sD to DRB NSSAI in case sD != 0xffffff

Currently, sD has not been allocated, so the octet string is empty. Fix the allocation to correctly write&read NSSAIs in UE context setup/modification request messages.

Before fixing oai_e2e_3_slices_ue_context_setup_no_nssai_sd_ngap_f1ap.pcapng

PDU Session Resource Setup Request message with sst:03 and sd:0xdef012

However, UE Context Setup Request message includes only sst:03

After fixing oai_e2e_3_slices_ue_context_setuphave_nssai_sd_ngap_f1ap.pcapng

PDU Session Resource Setup Request message with sst:03 and sd:0xdef012

UE Context Setup Request message includes sst:03 and sd:0xdef012