Clean up conversion warnings related to base64_{en, de}code

Clean up conversion warnings related to base64_{en, de}code

It seems unlikely that we can change the API at this point,
especially with the integration into the plugin API.

So
 - clean up the functions internally to not throw -Wconversion
   warnings
 - clean up any warnings on the caller side

Change-Id: Id7a5b2d8dea01bd532f5bcc8abea0e52b00d1169
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1148
Message-Id: <20250923103429.1257-1-gert@greenie.muc.de>
URL: https://sourceforge.net/p/openvpn/mailman/message/59237399/
Signed-off-by: Gert Doering <gert@greenie.muc.de>