filechooser: Fix crash when opening a file chooser with single filter…

… set

'gtk_file_filter_new_from_gvariant()' returns a 'GtkFileFilter' which
is a floating reference in GTK3.

Since the portal is taking ownership of the file filter through
'g_autoptr', we call 'g_object_ref_sink()' unconditionally when
creating the filter instance.

src/filechooser.c

@@ -557,7 +557,7 @@ handle_open (XdpImplFileChooser *object,
       g_autoptr (GtkFileFilter) filter = NULL;
       const char *current_filter_name;
 
-      filter = gtk_file_filter_new_from_gvariant (current_filter);
+      filter = g_object_ref_sink (gtk_file_filter_new_from_gvariant (current_filter));
       current_filter_name = gtk_file_filter_get_name (filter);
 
       if (!filters)