(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.

graph_gui.c

@@ -1132,10 +1132,8 @@ static void GlobalSortSignList(void)
 		error("Could not allocate memory for the sign-sorting-list");
 
 	FOR_ALL_SIGNS(ss) {
-		if (ss->str != STR_NULL) {
-			_sign_sort[n++] = ss->index;
-			_num_sign_sort++;
-		}
+		_sign_sort[n++] = ss->index;
+		_num_sign_sort++;
 	}
 
 	qsort(_sign_sort, n, sizeof(_sign_sort[0]), SignNameSorter);