p2p/kademlia: add nil guards to connWrapper and pool

[mateeullahmalik]

• What: Add nil-safety to connWrapper and prevent typed-nil wrappers entering the pool.
• Changes:
  • Guard Read/Write/Set Deadline against nil receiver/inner conns, return io.ErrClosedPipe.
  • Make Close/LocalAddr/RemoteAddr nil-safe.
  • Ignore typed-nil *connWrapper in ConnPool.Add.
• Why: Prevents "RARE" typed-nil interface cases from causing hangs or panics in Kademlia read/write paths.
• Impact: No behavior change for valid connections; safer failure mode for invalid wrappers. No public API changes.