[gnome-boxes] installer: Don't assume OS to be always known

[Zeeshan Ali Khattak <zeeshanak src gnome org>]

commit 9061bd34e39670d53439fe93250b67376d249982
Author: Zeeshan Ali (Khattak) <zeeshanak gnome org>
Date:   Tue May 21 17:34:16 2013 +0300

    installer: Don't assume OS to be always known
    
    This fixes regression intruduce by commit 82199fc (Disabling express
    install disables additional devices).
    
    https://bugzilla.gnome.org/show_bug.cgi?id=700477

 src/installer-media.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/installer-media.vala b/src/installer-media.vala
index 16698b4..8cda645 100644
--- a/src/installer-media.vala
+++ b/src/installer-media.vala
@@ -15,7 +15,7 @@ private class Boxes.InstallerMedia : GLib.Object {
 
     public virtual Osinfo.DeviceList supported_devices {
         owned get {
-            return os.get_all_devices (null);
+            return (os != null)? os.get_all_devices (null) : new Osinfo.DeviceList ();
         }
     }