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.
xmlCtxtReadFd(xmlParserCtxtPtr ctxt, int fd,
              const char *URL, const char *encoding, int options)
{
    xmlParserInputBufferPtr input;
    xmlParserInputPtr stream;

    if (fd < 0)
        return (NULL);
    if (ctxt == NULL)
        return (NULL);
    xmlInitParser();

    xmlCtxtReset(ctxt);


    input = xmlParserInputBufferCreateFd(fd, XML_CHAR_ENCODING_NONE);
    if (input == NULL)
        return (NULL);
    input->closecallback = NULL;
    stream = xmlNewIOInputStream(ctxt, input, XML_CHAR_ENCODING_NONE);
    if (stream == NULL) {
        xmlFreeParserInputBuffer(input);
        return (NULL);
    }
    inputPush(ctxt, stream);
    return (xmlDoRead(ctxt, URL, encoding, options, 1));
}

xmlCtxtReadFd(xmlParserCtxtPtr ctxt, int fd,
              const char *URL, const char *encoding, int options)
{
    xmlParserInputBufferPtr input;
    xmlParserInputPtr stream;

    if (fd < 0)
        return (NULL);
    if (ctxt == NULL)
        return (NULL);
    xmlInitParser();

    xmlCtxtReset(ctxt);


    input = xmlParserInputBufferCreateFd(fd, XML_CHAR_ENCODING_NONE);
    if (input == NULL)
        return (NULL);
    input->closecallback = NULL;
    stream = xmlNewIOInputStream(ctxt, input, XML_CHAR_ENCODING_NONE);
    if (stream == NULL) {
        xmlFreeParserInputBuffer(input);
        return (NULL);
    }
    inputPush(ctxt, stream);
    return (xmlDoRead(ctxt, URL, encoding, options, 1));
}
