listbox with empty pile as last element crashes

[mwhudson]

Simple reproducer:

mwhudson@aeglos:~/tmp$ cat ut.py
import urwid

def exit_on_q(input):
    if input in ('q', 'Q'):
        raise urwid.ExitMainLoop()

listbox = urwid.ListBox([
    urwid.Text("hi"),
    urwid.Pile([]),
    ])

loop = urwid.MainLoop(listbox, unhandled_input=exit_on_q)
loop.run()
mwhudson@aeglos:~/tmp$ python ut.py
Traceback (most recent call last):
  File "ut.py", line 13, in <module>
    loop.run()
  File "/usr/lib/python2.7/dist-packages/urwid/main_loop.py", line 278, in run
    self._run()
  File "/usr/lib/python2.7/dist-packages/urwid/main_loop.py", line 376, in _run
    self.event_loop.run()
  File "/usr/lib/python2.7/dist-packages/urwid/main_loop.py", line 682, in run
    self._loop()
  File "/usr/lib/python2.7/dist-packages/urwid/main_loop.py", line 710, in _loop
    self._entering_idle()
  File "/usr/lib/python2.7/dist-packages/urwid/main_loop.py", line 671, in _entering_idle
    callback()
  File "/usr/lib/python2.7/dist-packages/urwid/main_loop.py", line 564, in entering_idle
    self.draw_screen()
  File "/usr/lib/python2.7/dist-packages/urwid/main_loop.py", line 578, in draw_screen
    canvas = self._topmost_widget.render(self.screen_size, focus=True)
  File "/usr/lib/python2.7/dist-packages/urwid/widget.py", line 179, in finalize_render
    canv = fn(self, size, focus=focus)
  File "/usr/lib/python2.7/dist-packages/urwid/listbox.py", line 509, in render
    raise ListBoxError, "Listbox contents too short!  Probably urwid's fault (please report): %r" % ((top,middle,bottom),)
urwid.listbox.ListBoxError: Listbox contents too short!  Probably urwid's fault (please report): ((0, []), (0, <Text flow widget b'hi'>, 0, 1, None), (0, []))