org-set-effort: Fix void variable error

* lisp/org.el (org-set-effort): Check if org-clock-current-task is
  defined because org-clock.el may not be loaded yet.

lisp/org.el

@@ -15711,7 +15711,7 @@ When INCREMENT is non-nil, set the property to the next allowed value."
      '((effort . identity)
        (effort-minutes . org-duration-string-to-minutes))
      val)
-    (when (string= heading org-clock-current-task)
+    (when (string= heading (org-bound-and-true-p org-clock-current-task))
       (setq org-clock-effort (get-text-property (point-at-bol) 'effort))
       (org-clock-update-mode-line))
     (message "%s is now %s" prop val)))