- Bugfix.  When a theme does no longer exist on disk, it should not be
  returned by theme_list() as it breaks the site ...  The attached patch
  prevents this from happening but does not remove the theme from the system
  table in the SQL database - if the theme is removed from the database upon
  viewing the themes administration page (or another trigger), I guess that
  is fine.

Kjartan: my fix is the right thing to apply because theme_list() is used
         elsewhere where it requires to return a list of existing themes.
         Maybe it's a little sad but the current theme loading code might
         be slower and more complex than the old Drupal 3 theme loading
         code ... ?