Simplify the code in X509_get_pubkey_parameters(3)

Simplify the code in X509_get_pubkey_parameters(3)
by using X509_get0_pubkey(3) instead of X509_get_pubkey(3);
no functional change.

OK tb@

This is similar to the relevant part of the follwoing commit
from the OpenSSL 1.1.1 branch, which is still under a free licence,
but without the bug that commit introduced into this function in OpenSSL:

commit c01ff880d47392b82cce2f93ac4a9bb8c68f8cc7
Author: Dr. Stephen Henson <steve@openssl.org>
Date:   Mon Dec 14 13:13:32 2015 +0000