netsniff-ng: Don't modify optarg/argv

We shouldn't modify optarg (and thus argv) since it's e.g. used to
display the commandline string in `ps'. Since strtoul() reads until it
encounters the first non-numeric character and ignores the rest, we can
just revert from setting a NULL byte after the numeric part of the
string.

Reported-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>