MSDN says WM_MDISETMENU shouldn't change the frame menu if it is given
as NULL. Tests bear that out. Some other tests are included to show we
haven't broken behavior on e.g. initially maximized MDI child windows.
Signed-off-by: Andrew Eikum <aeikum@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>