[dwm] small cleanup of drawtext()

Hi,

I do small cleanup of drawtext():

- buf[] is not needed to be global
- there's no need to end buf[] with zero byte, as all function take length
  parameter
- w > dc.w can be true iff length of printed text is zero

-Ph

-- 
Premysl "Anydot" Hruby, http://www.redrum.cz/