org-vm.el: properly handle virtual folders.

This fix is inspired by comments from Arik Mitschang.

lisp/org-vm.el

@@ -57,8 +57,7 @@
   (when (and (or (eq major-mode 'vm-summary-mode)
 		 (eq major-mode 'vm-presentation-mode))
 	     (save-window-excursion
-	       (condition-case nil (vm-select-folder-buffer)
-		 (error "Buffer not associated with any file"))))
+	       (vm-select-folder-buffer) buffer-file-name))
     (and (eq major-mode 'vm-presentation-mode) (vm-summarize))
     (vm-follow-summary-cursor)
     (save-excursion