libxml2: add Debian patches to address CVEs

Debian uses libxml2 2.9.4 in Stretch. This adds their security related
fixes from 2.9.4+dfsg1-2.2+deb9u2 to LEDE's 17.01 release.

Fixed CVEs:

CVE-2016-4658
CVE-2016-5131
CVE-2017-0663
CVE-2017-15412
CVE-2017-7375
CVE-2017-7376
CVE-2017-9047
CVE-2017-9048
CVE-2017-9049
CVE-2017-9050

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>

libs/libxml2/patches/0004-Fix-comparison-with-root-node-in-xmlXPathCmpNodes.patch

@@ -0,0 +1,34 @@
+From a005199330b86dada19d162cae15ef9bdcb6baa8 Mon Sep 17 00:00:00 2001
+From: Nick Wellnhofer <wellnhofer@aevum.de>
+Date: Tue, 28 Jun 2016 14:19:58 +0200
+Subject: [PATCH] Fix comparison with root node in xmlXPathCmpNodes
+
+This change has already been made in xmlXPathCmpNodesExt but not in
+xmlXPathCmpNodes.
+---
+ xpath.c | 4 ++--
+ 1 file changed, 2 insertions(+), 2 deletions(-)
+
+diff --git a/xpath.c b/xpath.c
+index 751665b..d992841 100644
+--- a/xpath.c
++++ b/xpath.c
+@@ -3342,13 +3342,13 @@ xmlXPathCmpNodes(xmlNodePtr node1, xmlNodePtr node2) {
+      * compute depth to root
+      */
+     for (depth2 = 0, cur = node2;cur->parent != NULL;cur = cur->parent) {
+-	if (cur == node1)
++	if (cur->parent == node1)
+ 	    return(1);
+ 	depth2++;
+     }
+     root = cur;
+     for (depth1 = 0, cur = node1;cur->parent != NULL;cur = cur->parent) {
+-	if (cur == node2)
++	if (cur->parent == node2)
+ 	    return(-1);
+ 	depth1++;
+     }
+-- 
+2.10.1
+