[gnome-shell] background: don't leak wall clock when background changes

[Ray Strode <halfline src gnome org>]

commit 94b96f8d9771d776eca28da72904bb04989238d2
Author: Ray Strode <rstrode redhat com>
Date:   Fri Dec 15 10:05:34 2017 -0500

    background: don't leak wall clock when background changes
    
    The background code allocates a GnomeWallClock when its first created,
    but neglects to drop a reference to that clock at destroy time.
    The undestroyed clocks lead to a timerfd leak that eventually prevents
    the shell from functioning.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=791655

 js/ui/background.js |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/js/ui/background.js b/js/ui/background.js
index 1c2b83b..3ea7900 100644
--- a/js/ui/background.js
+++ b/js/ui/background.js
@@ -289,6 +289,8 @@ var Background = new Lang.Class({
             this._clock.disconnect(this._timezoneChangedId);
         this._timezoneChangedId = 0;
 
+        this._clock = null;
+
         if (this._prepareForSleepId != 0)
             LoginManager.getLoginManager().disconnect(this._prepareForSleepId);
         this._prepareForSleepId = 0;