Freeze break: gnome-shell: icon in logout dialog

[Owen Taylor <otaylor redhat com>]

https://bugzilla.gnome.org/show_bug.cgi?id=646032

When the user has not selected a face image (which is probably
the normal case), the logout dialog shows a mysterious empty rectangle.
As we do in the user account panel and in GDM, we should use a default
avatar image for this.

Patch is simple and is tested to fix the problem.

- Owen