Fix session id and initial timestamp not being preserved

Fix session id and initial timestamp not being preserved

In the initial state of checking whether an auth-token has been
validated, the check check if multi->auth_token is already set and
only then sets the value. This defeats the purpose and lead to always
a new auth-token with new session id and lifetime being generated when
the server restarts or the client reconnect to another server.

Patch V2: Only set multi->auth_token when NULL to avoid leaking
          memory. Improve comments and documentation of auth-token.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200512124344.15929-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19878.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>