Prepare the client certificate earlier

Move the preparation of the client certificate to be post processing work
after reading the CertificateRequest message rather than pre processing
work prior to writing the Certificate message. As part of preparing the
client certificate we may discover that we do not have one available. If
we are also talking SSLv3 then we won't send the Certificate message at
all. However, if we don't discover this until we are about to send the
Certificate message it is too late and we send an empty one anyway. This
is wrong for SSLv3.

Reviewed-by: Emilia Käsper <emilia@openssl.org>