Fix a Segfault on long labels

The program 'chsmack' was crashing with a 'segmentation fault'
when called with label longer than 255 characters, as, for example:
chsmack -a $(head -c256 /dev/zero | tr '\0' x) afile

The problem came from the function getopt_long and from how it was
used. Removing any reference to its 5th parameter (replaced with
NULL) solves the problem.

I also inserted a exit in case of error. This is better to my eyes
to abort any action in case of error than to perform only some,
as it was the case.