Allow no SSL_CERTIFICATE when using init_ssl callback.

I wanted to use the init_ssl callback to set up the CA certificates, certificate, and key, which are all in DER format not PEM. The function ssl_set_option() still required the SSL certificate option even when using the callback. The call to SSL_use_certificate_chain_file() is still skipped if the certificate option isn't set.

So, the initial check for the SSL certificate option also checks to see that the init_ssl callback is set if the pem is NULL.