[gnome-shell] background: fix early destroy of SystemBackground

[Giovanni Campagna <gcampagna src gnome org>]

commit cfef1071145629603fec7e124909e77545579e62
Author: Giovanni Campagna <gcampagna src gnome org>
Date:   Thu Jun 26 18:45:06 2014 +0200

    background: fix early destroy of SystemBackground
    
    If the SystemBackground is destroyed before loading, we call
    removeImageContent() with null, which crashes.

 js/ui/background.js |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/js/ui/background.js b/js/ui/background.js
index 3730beb..0212699 100644
--- a/js/ui/background.js
+++ b/js/ui/background.js
@@ -659,7 +659,10 @@ const SystemBackground = new Lang.Class({
     },
 
     _onDestroy: function() {
-        this._cache.removeImageContent(this.actor.content);
+        let content = this.actor.content;
+
+        if (content)
+            this._cache.removeImageContent(content);
     },
 });
 Signals.addSignalMethods(SystemBackground.prototype);