Fix: accept 255 character labels

Example:

 # /home/jsakkine/devel/smack/utils/chsmack \
   -a$(printf '12345%.0s' {1..51}) foo
 foo: Invalid argument
 # /home/jsakkine/devel/smack/utils/chsmack \
   -a$(printf '12345%.0s' {1..51} | head -c -1) foo
 #

This patch fixes this issue.
(cherry picked from commit 1794fed08418b0e254e1e3a7325a8a67f8186bbc)