Fix '--cipher none --cipher' crash

Fix '--cipher none --cipher' crash

As reported in trac #699, OpenVPN crashes when an "--cipher none" option
is followed by "--cipher" (without arguments).  Fix the crash, and print a
warning to indicate that using --cipher of --auth without an argument is
deprecated.

This is a (partly) backport of the patch I sent for the master branch
yesterday.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1469541433-1671-1-git-send-email-steffan.karger@fox-it.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/12107
Signed-off-by: Gert Doering <gert@greenie.muc.de>