Fix CVE-2017-0663:

https://bugzilla.gnome.org/show_bug.cgi?id=780228 (not yet public)
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-0663
https://security-tracker.debian.org/tracker/CVE-2017-0663

Patch copied from upstream source repository:

https://git.gnome.org/browse/libxml2/commit/?id=92b9e8c8b3787068565a1820ba575d042f9eec66

From 92b9e8c8b3787068565a1820ba575d042f9eec66 Mon Sep 17 00:00:00 2001
From: Nick Wellnhofer <wellnhofer@aevum.de>
Date: Tue, 6 Jun 2017 12:56:28 +0200
Subject: [PATCH] Fix type confusion in xmlValidateOneNamespace

Comment out code that casts xmlNsPtr to xmlAttrPtr. ID types on
namespace declarations make no practical sense anyway.

Fixes bug 780228.

Found with libFuzzer and ASan.
---
 valid.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/valid.c b/valid.c
index 8075d3a0..c51ea290 100644
--- a/valid.c
+++ b/valid.c
@@ -4627,6 +4627,12 @@ xmlNodePtr elem, const xmlChar *prefix, xmlNsPtr ns, const xmlChar *value) {
 	}
     }
 
+    /*
+     * Casting ns to xmlAttrPtr is wrong. We'd need separate functions
+     * xmlAddID and xmlAddRef for namespace declarations, but it makes
+     * no practical sense to use ID types anyway.
+     */
+#if 0
     /* Validity Constraint: ID uniqueness */
     if (attrDecl->atype == XML_ATTRIBUTE_ID) {
         if (xmlAddID(ctxt, doc, value, (xmlAttrPtr) ns) == NULL)
@@ -4638,6 +4644,7 @@ xmlNodePtr elem, const xmlChar *prefix, xmlNsPtr ns, const xmlChar *value) {
         if (xmlAddRef(ctxt, doc, value, (xmlAttrPtr) ns) == NULL)
 	    ret = 0;
     }
+#endif
 
     /* Validity Constraint: Notation Attributes */
     if (attrDecl->atype == XML_ATTRIBUTE_NOTATION) {
-- 
2.14.1