Allow TLSv1.3 EC certs to use compressed points

The spec does not prohib certs form using compressed points. It only
requires that points in a key share are uncompressed. It says nothing
about point compression for certs, so we should not fail if a cert uses a
compressed point.

Fixes #5743

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/6055)