Document ftp: stop early if not in a node.

This will rather help debug some cases, where we try operations on the
"/" path.

bzr revid: p_christ@hol.gr-20100702103858-8jrqfzkd8g65vrzr

addons/document_ftp/ftpserver/abstracted_fs.py

@@ -319,6 +319,7 @@ class abstracted_fs(object):
     def get_node_cr_uid(self, node):
         """ Get cr, uid, pool from a node
         """
+        assert node
         db = pooler.get_db(node.context.dbname)
         return db.cursor(), node.context.uid