polarssl: also allocate PKCS#11 certificate object on demand

polarssl: also allocate PKCS#11 certificate object on demand

Commit 444a93ea changed certificate allocation to be postponed until
actual usage to fix --client-cert-not-required / --verify-client-cert
for PolarSSL builds.  However, I forgot to allocate when using pkcs11
(because that code does not use the tls_ctx_load_cert_file() function).

And while we're at it, use ALLOC_OBJ_CLEAR() instead of malloc + manual
check.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1448101324-20310-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10543
Signed-off-by: Gert Doering <gert@greenie.muc.de>