Clients of the WebExtensions portal see the FDs as closed when the portal is used for the first time

[Impact]

 * Without this patch, the WebExtensions portal won't allow native connectors to communicate with a browser the first time they are spawned by the portal. Closing and restarting the browser won't do, only killing the portal and letting it restart.

 * This patch is being backported to jammy as this is currently the primary target for the effort of enabling native messaging in the firefox snap (not widely available yet, but the portal is available in jammy, and once the firefox implementation lands in the snap, we want this to work seamlessly).

 * The patch is trivial, it adds a missing variable initialization, which fixes the problem.

[Test Plan]

 * Steps to reproduce the bug:
   - in a fully up-to-date jammy VM, install the chrome-gnome-shell package, and download and side-load the latest test snap from https://launchpad.net/~osomon/+snap/firefox-native-messaging (because the firefox implementation hasn't landed upstream yet)
   - if it exists, delete ~/.local/share/flatpak/db/webextensions and reboot to ensure a pristine state
   - run the firefox snap, browse to https://addons.mozilla.org/firefox/addon/gnome-shell-integration/, and click the "Add to Firefox" blue button
   - accept the firefox prompt to install the extension
   - you will soon be prompted by GNOME Shell (with a modal dialog) to allow firefox to use the WebExtensions portal to start the native connector, click "Allow"
   - browse to https://extensions.gnome.org/local/
   - without the patch, you'll get a message that "an unexpected error occurred"
   - with the patch, firefox is able to talk to the native connector and displays a page with the currently installed GNOME Shell extensions

 * Note that this can also be tested with another extension that uses native messaging to talk to a native connector on the system, e.g. https://github.com/keepassxreboot/keepassxc-browser

[Where problems could occur]

 * The patch is trivial and it's hard to imagine that initializing an uninitialized variable could have undesirable side effects.

 * In any case, the WebExtensions portal isn't currently used by any piece of software (this is being tested for firefox but isn't merged and therefore not widely available yet), so possible side effects wouldn't be noticed until the native messaging portal support lands in the firefox snap (see https://phabricator.services.mozilla.com/D140803).

[Other Info]

 * We are still hoping to get the native messaging portal support in the firefox snap in time for Ubuntu 22.04.1, so for it to be a seamless experience this patch needs to land for the .1 release too.

[Original Description]

(this was initially reported here: https://github.com/flatpak/xdg-desktop-portal/pull/705#issuecomment-1123392120)

When testing the WebExtensions portal against the only known client so far (firefox snap + the corresponding upstream patch https://phabricator.services.mozilla.com/D140803), I'm consistently seeing the following problem: the first time the user is prompted for authorization and accepts, the portal will spawn the native connector and pass the file descriptors to the client (firefox snap), but the client sees the file descriptors as closed. Closing the client and re-opening it doesn't help. Only when the portal is terminated and restarted does communication through FDs start working normally.

It turns out this is caused by a missing variable initialization, which is trivially fixed by https://github.com/jhenstridge/xdg-desktop-portal/pull/2.

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: xdg-desktop-portal 1.14.4-1ubuntu1~22.04.1
ProcVersionSignature: Ubuntu 5.15.0-41.44-generic 5.15.39
Uname: Linux 5.15.0-41-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl icp
ApportVersion: 2.20.11-0ubuntu82.1
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: ubuntu:GNOME
Date: Wed Jul 20 14:03:15 2022
InstallationDate: Installed on 2020-09-16 (671 days ago)
InstallationMedia: Ubuntu 20.10 "Groovy Gorilla" - Alpha amd64 (20200910)
SourcePackage: xdg-desktop-portal
UpgradeStatus: Upgraded to jammy on 2022-03-19 (122 days ago)