[xmonad] darcs patch: Fix possible head []

[Spencer Janssen]

On Tue, Jan 06, 2009 at 08:21:15PM +0100, Joachim Breitner wrote:
> Hi,
> 
> there are not many places where head is used in xmonad, but this is one of
> them. Time to make it safe.
> 
> Greetings,
> Joachim

Applied.