libxml2: pick up upstream schema validation patch

Fixes https://github.com/Homebrew/homebrew-core/issues/5370.

Closes #8791.

Signed-off-by: Tomasz Pajor <tomek@polishgeeks.com>

Formula/libxml2.rb

@@ -1,7 +1,7 @@
 class Libxml2 < Formula
   desc "GNOME XML library"
   homepage "http://xmlsoft.org"
-  revision 1
+  revision 2
 
   stable do
     url "http://xmlsoft.org/sources/libxml2-2.9.4.tar.gz"
@@ -20,6 +20,12 @@ class Libxml2 < Formula
             "patches/0006-Disallow-namespace-nodes-in-XPointer-ranges.patch",
             "patches/0007-Fix-more-NULL-pointer-derefs-in-xpointer.c.patch"
     end
+
+    # https://bugzilla.gnome.org/show_bug.cgi?id=766834
+    patch do
+      url "https://git.gnome.org/browse/libxml2/patch/?id=3169602058bd2d04913909e869c61d1540bc7fb4"
+      sha256 "42082b0e7fa80eac68abeace98ea5a03e8cd44cd781c13966eb0758b9a1749b3"
+    end
   end
 
   bottle do