Disallow lameduck's float to an address taken by another client

Disallow lameduck's float to an address taken by another client

Existing check didn't take into account the case when floated client is
lame duck (CN for lame duck is NULL), which allowed lame duck to float
to an address taken by another client.

As a fix we use cert_hash_compare function which, besides fixing
mentioned case, also allows lame duck to float to an address already
taken by the same client.
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1420658798-29943-1-git-send-email-lstipakov@gmail.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9386

Signed-off-by: Gert Doering <gert@greenie.muc.de>