Fix "Symbol’s function definition is void: org-clocking-buffer"

* lisp/org.el (org-sort-entries): Fix "Symbol’s function definition is
void: org-clocking-buffer".

Reported-by: Ian Garmaise <ian.g@phorixsol.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-08/msg00647.html>

Reported-by: Allen Choong <allencch@hotmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-08/msg00649.html>

lisp/org.el

@@ -9141,7 +9141,7 @@ function is being called interactively."
 	     ;; The clock marker is lost when using `sort-subr'; mark
 	     ;; the clock with temporary `:org-clock-marker-backup'
 	     ;; text property.
-	     (when (and (eq (org-clocking-buffer) (current-buffer))
+	     (when (and (eq (org-clock-is-active) (current-buffer))
 			(<= start (marker-position org-clock-marker))
 			(>= end (marker-position org-clock-marker)))
 	       (org-with-silent-modifications