strongswan: add upstream security fix for CVE-2018-17540

The fix for CVE-2018-16151 / 16152 contained an exploitable bug.
For details, see:

https://www.strongswan.org/blog/2018/10/01/strongswan-vulnerability-(cve-2018-17540).html



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