Minimal diff that fixes build errors for trusty/6.1.2x
Created by: clonm
Trusty is still supported, but backporting from Vivid is not. This commit backports sympa 6.1.24 from xenial instead of 6.1.23 from vivid. also doesn't fail if /var/spool/nullmailer and /var/spool/sympa haven't been initialized yet.