userMenu: Don't show the default avatar over a newly selected one

When we replace the default avatar image with a real image, we need
to remove the default avatar image.

https://bugzilla.gnome.org/show_bug.cgi?id=657657

js/ui/userMenu.js

@@ -226,6 +226,7 @@ IMStatusChooserItem.prototype = {
 
     _setIconFromFile: function(iconFile) {
         this._iconBin.set_style('background-image: url("' + iconFile + '");');
+        this._iconBin.child = null;
     },
 
     _setIconFromName: function(iconName) {