crypto_epoch: Clean up type handling in ovpn_expand_label()

crypto_epoch: Clean up type handling in ovpn_expand_label()

- Add explicit casts where we have checked the value and
  need to put it into a smaller type.
- Adapt some types to actual usage.

Change-Id: Iad717f0ff3c79ae199c8be5f93bc51bf258c68c3
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: MaxF <max@max-fillinger.net>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1218
Message-Id: <20251004061545.7277-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59242119/
Signed-off-by: Gert Doering <gert@greenie.muc.de>