Check if pkcs11_cert is NULL before freeing it

Check if pkcs11_cert is NULL before freeing it

When running openvpn --show-tls with mbedtls, it showed a null pointer
error at the end because of this.

Signed-off-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20221123154912.28394-1-maximilian.fillinger@foxcrypto.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25530.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 19c64f16baebbce966d55c62135d1ef066f7c8c2)