pgw: Avoid access to unbound global variable

This happens if GTv2C failed prior:
"""
11:30:17.577738 15 PGW_Tests.ttcn:645 setverdict(fail): none -> fail reason: "Unexpected GTPv2 while waiting for CreateSessionResp", new component reason: "Une
xpected GTPv2 while waiting for CreateSessionResp"
11:30:17.578004 15 PGW_Tests.ttcn:756 Dynamic test case error: Initializing a TTCN_Buffer with an unbound octetstring value.
"""

Change-Id: I9731b755ddc7bb3ebd56bdec550961cc36f2e2c6
1 file changed