combobox: popdown() the menu during unmap()

combobox: popdown() the menu during unmap()

combo_box_popdown() currently skips popping down 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.

Even once we remove all the now-pointless NULL checks, destroy() was the
wrong place to call combo_box_popdown(), and unmap() is the right place.