fixes error when loading the game and encountering NULL townPtrs

As mentioned here: http://www.7kfans.com/forums/viewtopic.php?f=13&t=690&p=4692#p4692

When the game is first loading, the new TownArray::operator[]() runs before nation_array is loaded and if TownArray encounters an error and attempts to save the game, the gamesave will crash because it hasn't finished loading and there are uninitialized variables.