block-dns using iservice: fix a potential double free

block-dns using iservice: fix a potential double free

- An item added to undo-list was not removed on error, causing
  attempt to free again in Undo().
  Also fix a memory leak possibility in the same context.

Github: fixes OpenVPN/openvpn#232

v2: Split add and delete functions and reuse the delete
function for cleanup.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20230201170735.2266851-1-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26130.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>