pantheon.elementary-greeter: actually fix the crash

Fixed a use-after-free issue where logged_in_context is used in update_style().
There are several reports for this but upstream has no action for this so far during the 6.x cycle.
See the provided link for more details.

pkgs/desktops/pantheon/desktop/elementary-greeter/default.nix

@@ -47,6 +47,9 @@ stdenv.mkDerivation rec {
       src = ./hardcode-fallback-background.patch;
       default_wallpaper = "${nixos-artwork.wallpapers.simple-dark-gray.gnomeFilePath}";
     })
+    # https://github.com/NixOS/nixpkgs/issues/151609
+    # https://github.com/elementary/greeter/issues/578#issuecomment-1030746697
+    ./fix-crash.patch
     # Fix build with meson 0.61
     # https://github.com/elementary/greeter/pull/590
     (fetchpatch {

pkgs/desktops/pantheon/desktop/elementary-greeter/fix-crash.patch

@@ -0,0 +1,21 @@
+diff --git a/src/Cards/UserCard.vala b/src/Cards/UserCard.vala
+index 83df22c..8cd917e 100644
+--- a/src/Cards/UserCard.vala
++++ b/src/Cards/UserCard.vala
+@@ -42,6 +42,7 @@ public class Greeter.UserCard : Greeter.BaseCard {
+     private Gtk.Stack login_stack;
+     private Greeter.PasswordEntry password_entry;
+
++    private SelectionCheck logged_in;
+     private unowned Gtk.StyleContext logged_in_context;
+     private weak Gtk.StyleContext main_grid_style_context;
+     private weak Gtk.StyleContext password_entry_context;
+@@ -214,7 +215,7 @@ public class Greeter.UserCard : Greeter.BaseCard {
+         };
+         avatar_overlay.add (avatar);
+
+-        var logged_in = new SelectionCheck () {
++        logged_in = new SelectionCheck () {
+             halign = Gtk.Align.END,
+             valign = Gtk.Align.END
+         };