Fix regression with session cache use by clients

Commit d316cdcf

 introduced some extra
checks into the session-cache update procedure, intended to prevent
the caching of sessions whose resumption would lead to a handshake
failure, since if the server is authenticating the client, there needs to
be an application-set "session id context" to match up to the authentication
context.  While that change is effective for its stated purpose, there
was also some collatoral damage introduced along with the fix -- clients
that set SSL_VERIFY_PEER are not expected to set an sid_ctx, and so
their usage of session caching was erroneously denied.

Fix the scope of the original commit by limiting it to only acting
when the SSL is a server SSL.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5967)