auth_token/tls_crypt: fix usage of md_valid()

auth_token/tls_crypt: fix usage of md_valid()

With b39725cf ("Remove md_kt_t and change crypto API to use const char*")
the logic for validating ciphers and md algorithms has been changed.

We should now *always* use md_valid() when validating a digest alg.

At the same time, add '!' (negation) when validating the digest algorithm
in the tls-crypt code, in order to restore the proper logic.

Cc: Arne Schwabe <arne@rfc2549.org>
Fixes: b39725cf ("Remove md_kt_t and change crypto API to use const char*")
Reported-by: Richard T Bonhomme <tincantech@protonmail.com>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20220215123157.10615-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23793.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>