Always ensure that session->cipher is set

If we have deserialized the SSL_SESSION then in some circumstances the
session->cipher value is NULL. We were patching up in some places but not
in others. We should just do it as part of loading the SSL_SESSION.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2259)