Move ASSERT so external-key with OpenSSL works again

Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1385728360-32127-1-git-send-email-arne@rfc2549.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8069

Signed-off-by: Gert Doering <gert@greenie.muc.de>