False positive security update

[Ivan Kurnosov]

Running `/usr/lib/update-notifier/apt-check` returns `119;1`

If I patch the code to see what package is to be upgraded (eg in the `isSecurityUpgrade` function) - it shows `libpq5`.

The machine has nothing to do and had never ever installed postgresql client

And at the moment it's not installed either

```
$ apt policy libpq5
libpq5:
  Installed: (none)
  Candidate: 10.12-0ubuntu0.18.04.1
  Version table:
     10.12-0ubuntu0.18.04.1 500
        500 http://nz.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
     10.3-1 500
        500 http://nz.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
```

After debugging some more it looks like

```
            if not (depcache.marked_install(pkg) or
                    depcache.marked_upgrade(pkg)):
                continue
```

this is the condition to blame: I expect the predicate in parentheses to be false - as the package is not marker for install or upgrade.

[Ivan Kurnosov (zerkms)]

Upd: it's not a single unique case, I have a bunch of other machines (with different roles and a set of installed packages) and all them report `libpq5` as outdated (yet none of them have even it installed)