Avoid checking for NULL before calling free functions.  This gets
ssl.*free:
    ssl_sess_cert_free ssl_free ssl_excert_free ssl_cert_free
    SSL_free SSL_SRP_CTX_free SSL_SESSION_free SSL_CTX_free
    SSL_CTX_SRP_CTX_free SSL_CONF_CTX_free
Reviewed-by: NKurt Roeckx <kurt@openssl.org>

sessions in the external session cache might be removed.
Submitted by: "Nadav Har'El" <nyh@math.technion.ac.il>

PR: 547