Ignore the session when setting SNI in s_client

As per this comment:

https://github.com/openssl/openssl/issues/4496#issuecomment-337767145



Since the server is entitled to reject our session our ClientHello
should include everything that we would want if a full handshake were
to happen. Therefore we shouldn't use the session as a source of
information for setting SNI.

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/4738)