treeview: Don't emit cursor-changed in destruction

This is a workaround until we have a good idea on how we want to handle
signal disconnection before/during/after dispose and destroy vfunc
calls.

https://bugzilla.gnome.org/show_bug.cgi?id=671939

gtk/gtktreeview.c

@@ -13238,7 +13238,8 @@ gtk_tree_view_real_set_cursor (GtkTreeView     *tree_view,
                                            GTK_CELL_RENDERER_FOCUSED);
     }
 
-  g_signal_emit (tree_view, tree_view_signals[CURSOR_CHANGED], 0);
+  if (!gtk_widget_in_destruction (GTK_WIDGET (tree_view)))
+    g_signal_emit (tree_view, tree_view_signals[CURSOR_CHANGED], 0);
 }
 
 /**