Fix assertion error when using --cipher none

Some commits ago, the cipher mode checks were cleaned up to
remove code duplication (and fix the issue in #471), but broke
'--cipher none' (reported in #473). This commit fixes that.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <545DED2C.5070002@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9217
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 4e93e6dc88f4d904a4f2eb90140472a8d8fd68d0)