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)

xmlGetNamespace(xmlParserCtxtPtr ctxt, const xmlChar *prefix) {
 int i;

 if (prefix == ctxt->str_xml) return(ctxt->str_xml_ns);
 for (i = ctxt->nsNr - 2;i >= 0;i-=2)
 if (ctxt->nsTab[i] == prefix) {
 if ((prefix == NULL) && (*ctxt->nsTab[i + 1] == 0))
 return(NULL);
 return(ctxt->nsTab[i + 1]);
 }
 return(NULL);
}

xmlGetNamespace(xmlParserCtxtPtr ctxt, const xmlChar *prefix) {
 int i;

 if (prefix == ctxt->str_xml) return(ctxt->str_xml_ns);
 for (i = ctxt->nsNr - 2;i >= 0;i-=2)
 if (ctxt->nsTab[i] == prefix) {
 if ((prefix == NULL) && (*ctxt->nsTab[i + 1] == 0))
 return(NULL);
 return(ctxt->nsTab[i + 1]);
 }
 return(NULL);
}
