sortlistmodel: Fix handling of section sort keys

When the section sorter changes, we need to update
the keys, otherwise the sorter will continue to report
the old sections.

This code is currently a bit suboptimal, since the
creation of sort keys and section sort keys are
muddled together.

Fixes: #5854

gtk/gtksortlistmodel.c

@@ -912,6 +912,12 @@ gtk_sort_list_model_sorter_changed_cb (GtkSorter        *sorter,
             }
         }
 
+      if (self->section_sorter)
+        {
+          gtk_sort_keys_unref (self->section_sort_keys);
+          self->section_sort_keys = gtk_sorter_get_keys (self->section_sorter);
+        }
+
       if (gtk_sort_list_model_start_sorting (self, NULL))
         pos = n_items = 0;
       else