v0.38 stability improvements

Thanks to the recent crash log submissions, I managed to identify and fix two unpleasant issues:

- fixed a bug in graphics library which could crash the game when drawing a non-yet-initialized popup window

- improved the road generation procedure for tactical maps. Before, if map generator tried to put a road on the map, but failed to do it in a consistent way, the game crashed. It was not nice at all. Now it is fixed: if generator finds out it can't draw a consistent road, it abandons this idea and produces a map with no road on it. Furthermore, if it fails to detect that the road would be inconsistent, it just puts the road as it is, without crashing the game.

Your submissions do make the game better!