dovecot: add upstream security fix for CVE-2017-15132

dovecot: add upstream security fix for CVE-2017-15132

A flaw was found in dovecot 2.0 up to 2.2.33 and 2.3.0.  An abort of SASL
authentication results in a memory leak in dovecot's auth client used by
login processes.  The leak has impact in high performance configuration
where same login processes are reused and can cause the process to crash due
to memory exhaustion.

For more details, see:
http://www.openwall.com/lists/oss-security/2018/01/25/4

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>