package/dovecot: security bump to version 2.3.6

package/dovecot: security bump to version 2.3.6

Fixes
* CVE-2019-11494: Submission-login crashed with signal 11 due to null
  pointer access when authentication was aborted by disconnecting.
* CVE-2019-11499: Submission-login crashed when authentication was
  started over TLS secured channel and invalid authentication message
  was sent.

Release notes:
https://dovecot.org/pipermail/dovecot-news/2019-April/000408.html

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>