Really fix '--cipher none' regression

... by not incorrectly hinting to the compiler the function argument of
cipher_kt_mode_{cbc,ofb_cfb}() is nonnull, since that no longer is the
case.

Verified the fix on Debian Wheezy, one of the platforms the reporter in
trac #473 mentions with a compiler that would optimize out the required
checks.

Also add a testcase for --cipher none to t_lpback, to prevent further
regressions.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1417552920-31770-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9300
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 98156e90e1e83133a6a6a020db8e7333ada6156b)