disable DCO if no --dev was specified

disable DCO if no --dev was specified

The DCO logic is unable to proceed without --dev argument, therefore
just disable DCO if no --dev was specified by the user.

Right now, calling openvpn with DCO enabled (default) and no --dev
specified leads to a crash, because --dev is assumed to always be there.

Reported-by: Frank Lichtenheld <frank@lichtenheld.com>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220801150812.32561-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24772.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>