Fix connected explicit length underflow handling

[MrAlaskan]

Vulnerability Description

This change fixes an integer underflow issue in the connected explicit message handling path (see #568).

Fix Description

The fix adds an explicit minimum-length check in source/src/enet_encap/cpf.c before the sequence number is decoded.

For connected explicit packets:

• if data_item.length < 2, the packet is rejected immediately
• an error trace is emitted to make the reason visible during debugging
• no sequence number is read from an undersized payload
• malformed connected explicit requests are stopped at the CPF handling boundary

This keeps the fix localized to the actual faulting entry point and prevents malformed connected explicit packets from progressing further into the request processing path.