Fix "Symbol nil may not be buffer-local" error

* lisp/org.el (org-set-regexps-and-options): Fix "Symbol nil may not
  be buffer-local" error when encountering an nonexistent startup
  keyword.

Reported-by: Mike McLean <mike.mclean@pobox.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/92279>

lisp/org.el

@@ -4989,12 +4989,13 @@ related expressions."
 	(let ((startup (cdr (assq 'startup alist))))
 	  (dolist (option startup)
 	    (let ((entry (assoc-string option org-startup-options t)))
-	      (let ((var (nth 1 entry))
-		    (val (nth 2 entry)))
-		(if (not (nth 3 entry)) (org-set-local var val)
-		  (unless (listp (symbol-value var))
-		    (org-set-local var nil))
-		  (add-to-list var val))))))
+	      (when entry
+		(let ((var (nth 1 entry))
+		      (val (nth 2 entry)))
+		  (if (not (nth 3 entry)) (org-set-local var val)
+		    (unless (listp (symbol-value var))
+		      (org-set-local var nil))
+		    (add-to-list var val)))))))
 	;; TODO keywords.
 	(org-set-local 'org-todo-kwd-alist nil)
 	(org-set-local 'org-todo-key-alist nil)