Detect infinite recursion in parameter entities

When expanding a parameter entity in a DTD, infinite recursion could
lead to an infinite loop or memory exhaustion.

Thanks to Wei Lei for the first of many reports.

Fixes bug 759579.
xmlStopParser(xmlParserCtxtPtr ctxt) {
    if (ctxt == NULL)
        return;
    xmlHaltParser(ctxt);
    ctxt->errNo = XML_ERR_USER_STOP;
}

xmlStopParser(xmlParserCtxtPtr ctxt) {
    if (ctxt == NULL)
        return;
    xmlHaltParser(ctxt);
    ctxt->errNo = XML_ERR_USER_STOP;
}
