small fix for 3 digit mnc usage in sib1

When 3 digit MNC is used in PLMN, during preparation of SIB1, first digit of MNC was not taking into account. UE derives Serving Network Name (SNN) using PLMN info from sib1. Because of 5GCore and UE has different SNNs, this leads to XRES*, RES* mismatch during Authentication Procedure between 5GCore and UE. (SNN is used for derivation of these keys)