[dwm] fix for dmenu history pull request

Hi!

I stumbled across an array overflow in the dmenu history pull request:
dmenu sometimes crashed when the history file had less than HIST_SIZE
entries. The attached pull request fixes that.

The pull request also removes the blank lines that got written to the history
file when a dmenu selection was aborted.

Regards,
Christian

-- 
....Christian.Garbs.....................................http://www.cgarbs.de
Gib Gates keine Chance!