network: null-guard player_nr in nwc_auth_player_t

[janry]

    A peer-supplied player_nr can target a player slot the current game
    has not filled (slots 2..14 in the default starter map). The four
    deref sites in nwc_auth_player_t::execute trusted the slot and
    crashed the server with one 33-byte packet from any TCP-connected
    peer. Null-guard locally; the silent-fail path matches the existing
    "wrong password on a real slot" reply shape, so no wire change.

[prissi]

Thanks again, incorporated