Fix polarssl / mbedtls builds

Fix polarssl / mbedtls builds

Commit 8a399cd3 hardened the OpenSSL default cipher list,
but also introduced a change in shared code that causes
polarssl / mbedtls builds to break when no --tls-cipher is
specified.

This fix is backported code from the master branch.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1463122492-701-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11647
Signed-off-by: Gert Doering <gert@greenie.muc.de>