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.
xmlParseEntity(const char *filename) {
    return(xmlSAXParseEntity(NULL, filename));
}

xmlParseEntity(const char *filename) {
    return(xmlSAXParseEntity(NULL, filename));
}
