Improve fallback protection

A client that has fallen back could detect an inappropriate fallback if
the TLSv1.3 downgrade protection sentinels are present.

Fixes #6756

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6894)