When someones sends an invalid command e.g. `/k/k` (Two slashes, so that it also can be sent with pidgin.) in the lobby chat by using an external xmpp client, 0ad displays just a timestamp and nothing else. This patch fixes that behaviour by just ignoring invalid commands. Demonstrated without any comment or explanation to me by Hanniba_Baraq (same as @Hannibal_Barca ?).