xwm: Don't crash when setting selection with no seat

It's possible to set the clipboard with no seat present - one way is to
use the RDP backend and then run 'xclip -i -selection clipboard' locally
without making an RDP connection.

Check if seat is NULL to prevent this from crashing.

Fixes #698

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>
(cherry picked from commit bb993df2)