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)

Author: Brian C. Young

parser.c

@@ -7699,6 +7699,14 @@ xmlParsePEReference(xmlParserCtxtPtr ctxt)
 	    if (xmlPushInput(ctxt, input) < 0)
 		return;
 	} else {
+	    if ((entity->etype == XML_EXTERNAL_PARAMETER_ENTITY) &&
+	        ((ctxt->options & XML_PARSE_NOENT) == 0) &&
+	        ((ctxt->options & XML_PARSE_DTDVALID) == 0) &&
+	        ((ctxt->options & XML_PARSE_DTDLOAD) == 0) &&
+	        ((ctxt->options & XML_PARSE_DTDATTR) == 0) &&
+	        (ctxt->replaceEntities == 0) &&
+	        (ctxt->validate == 0))
+	        return;
 	    /*
 	     * TODO !!!
 	     * handle the extra spaces added before and after