fix(gtk): fix segfault on ctrl-d on older adw (#4971)

Ghostty with libadwaita older than 1.4.0 (in my case, that of debian
bookworm) will segfault when you exit ghostty with ctrl+d. This was
already fixed by #3694 but it re-appeared during a refactor. The fix
simply uses a reference to `tab.window.window` to call
`gtk_window_destroy`, because `tab.window` has already been [destroyed
by a signal
handler](6cbd69da78/src/apprt/gtk/Tab.zig (L130-L137))
triggered in the old libadwaita path.

Original issue: #3135

src/apprt/gtk/notebook_adw.zig

@@ -137,6 +137,8 @@ pub const NotebookAdw = struct {
 
         // If we have no more tabs we close the window
         if (self.nPages() == 0) {
+            const window = tab.window.window;
+
             // libadw versions <= 1.3.x leak the final page view
             // which causes our surface to not properly cleanup. We
             // unref to force the cleanup. This will trigger a critical
@@ -150,7 +152,7 @@ pub const NotebookAdw = struct {
 
             // `self` will become invalid after this call because it will have
             // been freed up as part of the process of closing the window.
-            c.gtk_window_destroy(tab.window.window);
+            c.gtk_window_destroy(window);
         }
     }
 };