bibrecord: sanity check in record_order_subfields
bibrecord: sanity check in record_order_subfields

* Adds check to prevent an error introduced by
  75a834c4b068e934ed89cb2824b79fd26bd42234 that occurs
  when a record is None.  (fixes #751)

tiborsimko/invenio,tiborsimko/invenio,inveniosoftware/invenio,inveniosoftware/invenio