org-vm.el: check if buffer refers to a file when storing a link.

* org-vm.el (org-vm-store-link): Make sure the buffer is
associated with a file when trying to store an Org link.

lisp/org-vm.el

@@ -54,8 +54,11 @@
 ;; Implementation
 (defun org-vm-store-link ()
   "Store a link to a VM folder or message."
-  (when (or (eq major-mode 'vm-summary-mode)
-	    (eq major-mode 'vm-presentation-mode))
+  (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"))))
     (and (eq major-mode 'vm-presentation-mode) (vm-summarize))
     (vm-follow-summary-cursor)
     (save-excursion