Fix out-of-bounds read in GtpV2Layer with truncated TEID-present header

[vsaraikin]

Fixes #2171.

GtpV2Layer::isDataValid only requires sizeof(gtpv2_basic_header) + sizeof(uint32_t) bytes (8 bytes). That is enough for the basic header plus the sequence-number word of a TEID-absent header, but when the TEID-present flag is set the fixed header additionally carries the 4-byte TEID before the sequence word:

• TEID absent: [basic header (4)][seq word (4)] → 8 bytes
• TEID present: [basic header (4)][TEID (4)][seq word (4)] → 12 bytes

So a TEID-present packet of only 8 bytes passes validation, and getSequenceNumber() (which advances an extra 4 bytes when teidPresent is set) then reads a 32-bit word past the buffer — the OOB read reported in the issue (PoC 48200008deadbeef, version 2 + TEID-present, 8 bytes).

The fix makes isDataValid require the extra 4 bytes when the TEID-present flag is set, so a truncated TEID-present header is rejected before any accessor can read past the buffer. The flag lives in the basic header, which is already covered by the existing minimum-length check, so reading it here is safe.

Testing

• Added a regression case to GtpV2LayerParsingTest: the 8-byte TEID-present PoC is now rejected by isDataValid, while the same header with the sequence word present (12 bytes) is accepted.
• Verified under AddressSanitizer: the new GtpV2 tests pass clean with the fix; without the fix isDataValid accepts the truncated packet (the assertion fails, and the underlying getSequenceNumber read is OOB).
• Full Packet++Test suite passes (258/258).