DO NOT MERGE: Add validation for eternal enities

https://bugzilla.gnome.org/show_bug.cgi?id=780691

Bug: 36556310
Change-Id: I9450743e167c3c73af5e4071f3fc85e81d061648
(cherry picked from commit bef9af3d89d241bcb518c20cba6da2a2fd9ba049)

inputPop(xmlParserCtxtPtr ctxt)
{
    xmlParserInputPtr ret;

 if (ctxt == NULL)
 return(NULL);
 if (ctxt->inputNr <= 0)
 return (NULL);
    ctxt->inputNr--;
 if (ctxt->inputNr > 0)
        ctxt->input = ctxt->inputTab[ctxt->inputNr - 1];
 else
        ctxt->input = NULL;
    ret = ctxt->inputTab[ctxt->inputNr];
    ctxt->inputTab[ctxt->inputNr] = NULL;
 return (ret);
}

inputPop(xmlParserCtxtPtr ctxt)
{
    xmlParserInputPtr ret;

 if (ctxt == NULL)
 return(NULL);
 if (ctxt->inputNr <= 0)
 return (NULL);
    ctxt->inputNr--;
 if (ctxt->inputNr > 0)
        ctxt->input = ctxt->inputTab[ctxt->inputNr - 1];
 else
        ctxt->input = NULL;
    ret = ctxt->inputTab[ctxt->inputNr];
    ctxt->inputTab[ctxt->inputNr] = NULL;
 return (ret);
}
