Fix `org-refile-cache-get' error.

This patch fixes the problem first reported by Tassilo Horn in
[mid:87y6fhxc47.fsf@thinkpad.tsdh.de].  Problem was that
`org-refile-cache-get' returned an invalid refile target table after
the refile cache was cleared.

lisp/ChangeLog

@@ -1,3 +1,8 @@
+2010-05-19  David Maus  <dmaus@ictsoc.de>
+
+	* org.el (org-refile-cache-get): Return empty list of targets
+	when cache was cleared.
+
 2010-05-17  Carsten Dominik  <carsten.dominik@gmail.com>
 
 	* org.el (org-refile-use-cache): New option.

lisp/org.el

@@ -9526,7 +9526,7 @@ on the system \"/user@host:\"."
   "Retrieve the cached value for refile targets given by IDENTIFIERS."
   (cond
    ((not org-refile-cache) nil)
-   ((not org-refile-use-cache) (org-refile-cache-clear))
+   ((not org-refile-use-cache) (org-refile-cache-clear) nil)
    (t
     (let ((set (cdr (assoc (sha1 (prin1-to-string identifiers))
 			   org-refile-cache))))