filechooser: Prevent a crash

With csd, we are handling external widgets when
there is an entry in the headerbar. Use a weak ref
to prevent that pointer from going stale. This fixes
a crash when cancelling a save dialog.

Fixes: #3110

gtk/gtkfilechooserwidget.c

@@ -2322,6 +2322,7 @@ save_widgets_create (GtkFileChooserWidget *impl)
     {
       location_entry_disconnect (impl);
       impl->location_entry = impl->external_entry;
+      g_object_add_weak_pointer (impl->external_entry, (gpointer *)&impl->location_entry);
       location_entry_setup (impl);
 
       g_signal_connect_after (gtk_entry_get_key_controller (GTK_ENTRY (impl->external_entry)),