Handle NULL intp in test_increment_intp()
Handle NULL intp in test_increment_intp()

Support passing NULL for &int to gtk_test_display_button_window() so
we can use that function also when we are not interested in counting
clicks.

johne53/MB3Gtk-2,Distrotech/gtk,Unity-Technologies/gtk,Distrotech/gtk,Unity-Technologies/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk,johne53/MB3Gtk-2,Distrotech/gtk,Distrotech/gtk,Unity-Technologies/gtk,Unity-Technologies/gtk