Segfault if crontab is owned by a user who cannot be found?

[Sitsofe Wheeler]

Description of the problem
It appears that cron will go on to crash if it finds a crontab owned by uid who does not map to a valid user.

Steps to reproduce:
1. Create a crontab with an Active Directory user set up via SSSD?
2. Switch AD domains so user's uid is entirely different?
3. Wait for cron to try and run the old user's crontab?

Expected result:
Error in the journal?

Actual result:
Error in the log file and cron segfault?

How reproducible is the issue?
It has happened twice so far before the crontab was removed.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: cron 3.0pl1-128ubuntu2
ProcVersionSignature: Ubuntu 4.4.0-103.126-generic 4.4.98
Uname: Linux 4.4.0-103-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2.14
Architecture: amd64
Date: Fri Dec 15 14:33:45 2017
InstallationDate: Installed on 2017-03-09 (280 days ago)
InstallationMedia: Ubuntu-Server 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215.8)
ProcEnviron:
 LANGUAGE=en_GB:en
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: cron
UpgradeStatus: No upgrade log present (probably fresh install)