(svn r6047) -Codechange: FOR_ALL now _only_ loops valid items, and skips invalid ones

-Codechange: use IsValidXXX where ever possible
  Note: both changes to prepare for new pool system, which needs those changes.
  For every pool there are 2 ugly lines, which will be removed when done
  implementing new pool system.
  Based on FS#13 by blathijs, partly implemented.

settings.c

@@ -1074,9 +1074,8 @@ static int32 PopulationInLabelActive(int32 p1)
 {
 	Town* t;
 
-	FOR_ALL_TOWNS(t) {
-		if (t->xy != 0) UpdateTownVirtCoord(t);
-	}
+	FOR_ALL_TOWNS(t) UpdateTownVirtCoord(t);
+
 	return 0;
 }