Always require an advertised NewSessionTicket message.

The server must send a NewSessionTicket message if it advertised one
in the ServerHello, so make a missing ticket message an alert
in the client.

An equivalent change was independently made in BoringSSL, see commit
6444287806d801b9a45baf1f6f02a0e3a16e144c.

Reviewed-by: Matt Caswell <matt@openssl.org>