fix crash when zomming in main menu

changelog

@@ -46,6 +46,7 @@ SVN trunk (1.1+svn):
    * improved (underwater) merfolk village
    * (internal) make attack animations work like all other animations
    * backport latest SDL_ttf change, fix ascender/descender issue for DejaVu
+   * fix crash when using zoom keys in the main lobby
 
 Version 1.1:
  * campaign server

src/display.cpp

@@ -63,6 +63,7 @@ namespace {
 
 	int MinZoom(const gamemap& map, const SDL_Rect& viewport)
 	{
+		if(map.x()<4 || !map.y() ) return DefaultZoom;
 		const int min_zoom1 = viewport.w/((map.x()*3)/4);
 		const int min_zoom2 = viewport.h/map.y();
 		return maximum<int>(min_zoom1,min_zoom2);