[xml] Bug in xpath.c

[François Delyon <f delyon satimage fr>]

Noticed in xmlXPathNextDescendantOrSelfElemParent:

#ifdef LIBXML_DOCB_ENABLED
                /* Not sure if we need those here. */
                case XML_DOCUMENT_NODE:
                case XML_DOCB_DOCUMENT_NODE:
#endif
                case XML_HTML_DOCUMENT_NODE:

This makes xpath requests like "//a" to fail under some circumstances  
when LIBXML_DOCB_ENABLED is 0.

I guess that the intent of the writer was:

                case XML_DOCUMENT_NODE:
#ifdef LIBXML_DOCB_ENABLED
                case XML_DOCB_DOCUMENT_NODE:
#endif
                case XML_HTML_DOCUMENT_NODE:


F.Delyon