combobox: popdown() the menu during unmap()
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.
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