fix a bug in org-open-at-point

My bad, wrong manipulation. Here is the patch.

To reproduce the error, you can type the following in a fresh Org
#+begin_src org
target some text <<<target>>> another text target
#+end_src

Using C-c C-o on any of the two links will return an error.
Regards,

>From f7738f3e9239fc4fddccc7850dad7a0936087a58 Mon Sep 17 00:00:00 2001
From: Nicolas Goaziou <n.goaziou@gmail.com>
Date: Sat, 19 Feb 2011 09:37:48 +0100
Subject: [PATCH] Fix bug with link to radio target at beginning or end of buffer

* lisp/org.el (org-open-at-point): if a link to a radio target is the
  first, (resp. the last), element of a buffer, function cannot find
  the property change required to get its boundaries, and
  `buffer-substring' is called with an invalid nil argument.

lisp/org.el

@@ -9218,8 +9218,10 @@ application the system uses for this file type."
 		pos (if (get-text-property (1+ (point)) 'org-linked-text)
 			(1+ (point)) (point))
 		path (buffer-substring
-		      (previous-single-property-change pos 'org-linked-text)
-		      (next-single-property-change pos 'org-linked-text)))
+		      (or (previous-single-property-change pos 'org-linked-text)
+			  (point-min))
+		      (or (next-single-property-change pos 'org-linked-text)
+			  (point-max))))
 	  (throw 'match t))
 
 	(save-excursion