Do not access internals of ASN1_INTEGER to print hex of serial

Do not access internals of ASN1_INTEGER to print hex of serial

OpenSSL 4.0 does not allow internal access to to these data structures
anymore. So use public methods to get the serial data and convert it to
hex.

Change-Id: I5158fbb0762443ea4954e5745f520e83e019ed30
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1589
Message-Id: <20260404155726.7696-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg36459.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 59934618e74f825806be3d190b6d3a98d182cd47)