* xmlschemas.c: Fixed bug #341337, reported by David Grohmann.
  The code expected a node (xmlNodePtr) on the info for a
  non-existent default attribute, which clearly cannot be
  expected, since the attribute does not exist. I can only
  guess that this sneaked trying to eliminate the query
  for the owner-element, which is unavoidable actually.
  Note that creation of default attributes won't have an
  effect if validating via SAX/XMLReader; i.e., the processor
  won't fire additional start-attribute events (I'm not even
  sure if Libxml2 has such a SAX-event; I think it hands them
  all over in the start-element event).