Backported from X.org:

 commit ae67508392261ae47858692668a0c192ef4a9d7b
 Author: Mathieu Bérard <mathieu.berard@crans.org>
 Date:   Fri Jul 25 10:05:30 2008 -0400

    Fix AddExtension now that CloseDownProc can be NULL.

 commit 6bcde69585fcc8f8dbfe81c115649f19274922fa
 Author: Adam Jackson <ajax@redhat.com>
 Date:   Thu Jul 24 09:13:26 2008 -0400

    Allow extension closedown hook to be null.

Authors: Adam Jackson <ajax@redhat.com>
         Mathieu Bérard <mathieu.berard@crans.org>
Backport to nx-libs: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>