Do not raise exception on close() on account of socket attribute still being None:

>>> import asyncore
>>> d = asyncore.dispatcher()
>>> d.close()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.4/asyncore.py", line 401, in close
    self.socket.close()
AttributeError: 'NoneType' object has no attribute 'close'
>>>