cmd/devp2p: fixes for eth and discv4 tests (#23155)

This PR fixes a false positive PONG 'to' endpoint mismatch seen in hive tests:

    got {IP:172.17.0.7 UDP:44025 TCP:44025}, want {IP:172.17.0.7 UDP:44025 TCP:0}

Co-authored-by: Felix Lange <fjl@twurst.com>