lib: GTP_Emulation: Allow receiving packets with TEID 0

Some GTP messages like Echo Request, Echo Reply and Ind Error don't use
the TEID value. According to 3GPP TS 29.060 sec 9.3.1 in those cases the TEID is
set to 0:

"""
- TEID: Contains the Tunnel Endpoint Identifier for the tunnel to which this T-PDU belongs. The TEID shall be
used by the receiving entity to find the PDP context, except for the following cases:
- The Echo Request/Response and Supported Extension Headers notification messages, where the Tunnel
Endpoint Identifier shall be set to all zeroes.
- The Error Indication message where the Tunnel Endpoint Identifier shall be set to all zeros.
"""

Change-Id: Ic702b78028e850ed961ef805f35e10a42da34e56
1 file changed