Use ``0'' instead of ``None'' to reset the underlying object in close

Use ``0'' instead of ``None'' to reset the underlying object in close
methods.  Using None causes problems if the destructor is called after
the __builtin__ module has already been destroyed (unfortunately, this
can happen!).  I can't just delete the object because it is actually
tested for (if self._sock: ...).  Setting it to 0 is a bit weird but
works.