Replace size check with more meaningful pubkey check

Replace size check with more meaningful pubkey check

It does not make sense to check the size because this
function can be used in other contexts than in TLS-1.3 and
the value might not be padded to the size of p.

However it makes sense to do the partial pubkey check because
there is no valid reason having the pubkey value outside the
1 < pubkey < p-1 bounds.

Fixes #15465

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17630)