dovecot: denial of service

Dovecot <= 2.2.14 does not correctly handle SSL/TLS handshake failure in
the login process, asking OpenSSL to flush a connection that has already
been aborted. This results in a crash with some versions of OpenSSL
(most likely >= 1.0.2). A patch to OpenSSL has also been written to
handle more gracefully this situation, see references.