combo_box_popdown() of course doesn’t popdown our menu if it is NULL.
But the required call to this at end-of-life was in destroy(), by which
point dispose() already NULLed the menu, so Menu::popdown() would never
run, even if it should. Fix this by trying popdown() earlier in unmap().
Also, add a converse assurance that we don’t popup() while not mapped.