Fix regression in setups without a client certificate

Fix regression in setups without a client certificate

This fixes a null-pointer dereference in tls_ctx_cert_time(), which will
occur on clients that do not use a client certificate (ie that only have
auth-user-pass in the config, but no key and cert).  This bug was
introduced by commit 091edd8e on the master branch, and commit dfd940bb
on the release/2.3 branch.

This bug was found by chipitsine and reported in trac ticket #644.

While touching this function, I also made this function conform to the
openvpn coding style.

v2 - fix memory leak in builds using pre-1.0.2 openssl

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1451814476-32574-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10921
Signed-off-by: Gert Doering <gert@greenie.muc.de>