exim: add upstream security fix

exim: add upstream security fix

Fixes the following security issue:

CVE-2018-6789: Meh Chang discovered a buffer overflow flaw in a utility
function used in the SMTP listener of Exim, a mail transport agent.  A
remote attacker can take advantage of this flaw to cause a denial of
service, or potentially the execution of arbitrary code via a specially
crafted message.

Dropped ChangeLog hunk and adjusted file path of upstream commit so it
applies to tarball.

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