dhcp: Fix conversion warnings

dhcp: Fix conversion warnings

- Use correct type for pointer difference
- Make sure that small sizeof sum is treated as int

Change-Id: Ie0c0fbf4f7f8b379d46b6755c4eff209acc20fef
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1241
Message-Id: <20251006210127.28679-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59243110/
Signed-off-by: Gert Doering <gert@greenie.muc.de>