Deactivate the timer when we destroy the puzzle window. Otherwise it

might run again in mid-shutdown and cause chaos, if you hit `q' in
the middle of an animated sequence such as the Net finishing flash.

[originally from svn r4525]

gtk.c

@@ -415,6 +415,8 @@ void activate_timer(frontend *fe)
 
 static void window_destroy(GtkWidget *widget, gpointer data)
 {
+    frontend *fe = (frontend *)data;
+    deactivate_timer(fe);
     gtk_main_quit();
 }