wesnothd: avoid invalid iterator use when terminating game

Set game::owner_ to past-the-end iterator when game is being destroyed
to avoid comparison with invalid iterator when remove_player() is called
for other players after game host has disconnected making game::owner_
invalid.

src/server/wesnothd/game.cpp

@@ -1450,6 +1450,7 @@ bool game::remove_player(player_iterator player, const bool disconnect, const bo
 	}
 
 	if(game_ended || destruct) {
+		owner_ = player_connections_.end();
 		return game_ended;
 	}