tls-crypt-v2: fix server memory leak

tls-crypt-v2: fix server memory leak

tls-crypt-v2 was developed in parallel with the changes that allowed to
use tls-auth/tls-crypt in connection blocks. The tls-crypt-v2 patch set
was never updated to the new reality after commit 5817b49b, causing a
memory leak of about 600 bytes for each connecting client.

It would be nicer to not reload the tls-crypt-v2 server key for each
connecting client, but that requires more refactoring (and thus more time
to get right). So for now just plug the leak by free'ing the memory when
we close a client connection.

To test this easily, compile openvpn with -fsanity=address, run a server
with tls-crypt-v2, connect a client, stop the server.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20201203182230.33552-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21310.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit fb169c3b8fdfa9792c0eee8441956f062dfd7982)