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

@@ -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;