To roll-back you can run this, as root:
apt-get install dovecot-core=1:2.2.13-12~deb8u1 dovecot-imapd=1:2.2.13-12~deb8u1 dovecot-ldap=1:2.2.13-12~deb8u1 dovecot-lmtpd=1:2.2.13-12~deb8u1 dovecot-managesieved=1:2.2.13-12~deb8u1 dovecot-pop3d=1:2.2.13-12~deb8u1 dovecot-sieve=1:2.2.13-12~deb8u1 dovecot-solr=1:2.2.13-12~deb8u1
That should always work, even if you don't have the old package(s) cached locally. You can do the same for other dovecot-packages, specifying the version number of the previous release (
1:2.2.13-12~deb8u1), but the
dovecot-core is the package that will resolve the problem.
Of course tomorrow the security fix will be automatically re-installed, which means you'll need to repeat the process. Or pin the package to prevent the upgrade from happening.