Unattended Upgrades failing with variable reference error

[Knut Ahlers]

When running unattended-upgrades with auto-remove enabled the script complains about a variable being referenced before assignment:

```
# unattended-upgrade -d
[...]
Removing unused kernel packages: linux-image-4.15.0-32-generic linux-modules-4.15.0-32-generic
Keeping 2 auto-removable package(s) because it would also remove the following packages which should be kept in this step: [...]
Traceback (most recent call last):
  File "/usr/bin/unattended-upgrade", line 1991, in <module>
    sys.exit(main(options))
  File "/usr/bin/unattended-upgrade", line 1791, in main
    options.verbose or options.debug, options.dry_run)
  File "/usr/bin/unattended-upgrade", line 1480, in do_auto_remove
    if res:
UnboundLocalError: local variable 'res' referenced before assignment
```

This causes the apt-daily-upgrade.service to "fail" which then again causes no updates to be installed, which means the package does not fulfil its purpose:

```
# systemctl status apt-daily-upgrade.service
● apt-daily-upgrade.service - Daily apt upgrade and clean activities
   Loaded: loaded (/lib/systemd/system/apt-daily-upgrade.service; static; vendor preset: enabled)
   Active: inactive (dead) since Sat 2018-10-20 06:17:09 CEST; 6h ago
     Docs: man:apt(8)
 Main PID: 32708 (code=exited, status=0/SUCCESS)

Oct 20 06:16:00 myhost systemd[1]: Starting Daily apt upgrade and clean activities...
Oct 20 06:17:09 myhost apt.systemd.daily[32708]: Traceback (most recent call last):
Oct 20 06:17:09 myhost apt.systemd.daily[32708]: File "/usr/bin/unattended-upgrade", line 1991, in <module>
Oct 20 06:17:09 myhost apt.systemd.daily[32708]: sys.exit(main(options))
Oct 20 06:17:09 myhost apt.systemd.daily[32708]: File "/usr/bin/unattended-upgrade", line 1791, in main
Oct 20 06:17:09 myhost apt.systemd.daily[32708]: options.verbose or options.debug, options.dry_run)
Oct 20 06:17:09 myhost apt.systemd.daily[32708]: File "/usr/bin/unattended-upgrade", line 1480, in do_auto_remove
Oct 20 06:17:09 myhost apt.systemd.daily[32708]: if res:
Oct 20 06:17:09 myhost apt.systemd.daily[32708]: UnboundLocalError: local variable 'res' referenced before assignment
Oct 20 06:17:09 myhost systemd[1]: Started Daily apt upgrade and clean activities.

# apt-get --just-print upgrade | grep ^Inst | wc -l
99
```

Mapped to the current source code of the script within the bionic branch the error occurs in this line:

https://git.launchpad.net/ubuntu/+source/unattended-upgrades/tree/unattended-upgrade?h=ubuntu/bionic&id=165a91cd44a77a03a83c64cd3812831ba26e3a80#n1418

Used version of the package:

```
# apt-cache policy unattended-upgrades
unattended-upgrades:
  Installed: 1.1ubuntu1.18.04.5
  Candidate: 1.1ubuntu1.18.04.5
  Version table:
 *** 1.1ubuntu1.18.04.5 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main i386 Packages
        500 http://mirror.hetzner.de/ubuntu/packages bionic-updates/main amd64 Packages
        500 http://mirror.hetzner.de/ubuntu/packages bionic-updates/main i386 Packages
        100 /var/lib/dpkg/status
     1.1ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages
        500 http://archive.ubuntu.com/ubuntu bionic/main i386 Packages
        500 http://mirror.hetzner.de/ubuntu/packages bionic/main amd64 Packages
        500 http://mirror.hetzner.de/ubuntu/packages bionic/main i386 Packages
```