gb_proxy: Introduce more validation / constraint checks

* ensure the BSSGP PDU header length before reading pdu_type field
* ensure we never process uplink PDUs in downlink and vice-versa
* ensure we never proceses PTP PDUs on SIGNALING BVCI and vice-versa

Change-Id: I6e40aed0283f1a0860ab273606605f7fb28717cf
Depends: libosmocore.git I7e4226463f3c935134b5c2c737696fbfd1dd5815
1 file changed