lisp/org.el: Fix org-update-statistics-cookies error

* lisp/org.el (org-update-statistics-cookies): Replace
usage of `org-map-entries' with `org-map-region' to prevent bug where
an error was thrown if the current buffer had not yet been saved to disk.

Formatted from a patch proposal by mail@kisaragi-hiu.com
Reported-by: mail@kisaragi-hiu.com

lisp/org.el

@@ -10012,7 +10012,7 @@ all statistics cookies in the buffer."
   (if all
       (progn
 	(org-update-checkbox-count 'all)
-	(org-map-entries 'org-update-parent-todo-statistics))
+	(org-map-region 'org-update-parent-todo-statistics (point-min) (point-max)))
     (if (not (org-at-heading-p))
 	(org-update-checkbox-count)
       (let ((pos (point-marker))