Bug#1059540: apt: Infinite loop when config file is a directory

[as@php.net]

Package: apt
Version: 2.6.1
Severity: normal
X-Debbugs-Cc: none

The following causes apt-get to go into an infinite busy loop:

```
$ mkdir /tmp/empty
$ apt-get -c /tmp/empty
```

This affects the latest dev build as well.

I submitted a fix here:
https://salsa.debian.org/apt-team/apt/-/merge_requests/314

-- System Information:
Debian Release: 12.2
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 6.1.0-13-amd64 (SMP w/8 CPU threads; PREEMPT)
Kernel taint flags: TAINT_PROPRIETARY_MODULE, TAINT_OOT_MODULE,
TAINT_UNSIGNED_MODULE
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
(ignored: LC_ALL set to en_US.UTF-8), LANGUAGE=en_US.UTF-8
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages apt depends on:
ii  adduser                 3.134
ii  debian-archive-keyring  2023.3+deb12u1
ii  gpgv                    2.2.40-1.1
ii  libapt-pkg6.0           2.6.1
ii  libc6                   2.36-9+deb12u3
ii  libgcc-s1               12.2.0-14
ii  libgnutls30             3.7.9-2+deb12u1
ii  libseccomp2             2.5.4-1+b3
ii  libstdc++6              12.2.0-14
ii  libsystemd0             252.17-1~deb12u1

Versions of packages apt recommends:
ii  ca-certificates  20230311

Versions of packages apt suggests:
pn  apt-doc         <none>
ii  dpkg-dev        1.21.22
ii  gnupg           2.2.40-1.1
pn  powermgmt-base  <none>
ii  synaptic        0.91.3