Apply behaviour change fix from upstream for previous XPath change.

BUG=58731
TEST=NONE

Review URL: http://codereview.chromium.org/4027006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@63572 0039d316-1c4b-4281-b951-d872f2087c98
xmlXPathRoot(xmlXPathParserContextPtr ctxt) {
    if ((ctxt == NULL) || (ctxt->context == NULL))
	return;
    ctxt->context->node = (xmlNodePtr) ctxt->context->doc;
    valuePush(ctxt, xmlXPathCacheNewNodeSet(ctxt->context,
	ctxt->context->node));
}

xmlXPathRoot(xmlXPathParserContextPtr ctxt) {
    if ((ctxt == NULL) || (ctxt->context == NULL))
	return;
    ctxt->context->node = (xmlNodePtr) ctxt->context->doc;
    valuePush(ctxt, xmlXPathCacheNewNodeSet(ctxt->context,
	ctxt->context->node));
}
