STREAM event can have a zero port
Now that there is some event handling, I've begun trying to develop against Stem and I found this crash: ``` File "/usr/lib64/python2.7/site-packages/stem/response/events.py", line 55, in _parse_message self._parse() File "/usr/lib64/python2.7/site-packages/stem/response/events.py", line 459, in _parse raise stem.ProtocolError("Source location's port is invalid: %s" % self) ProtocolError: Source location's port is invalid: STREAM 14 NEW 0 176.28.51.238.$649F2D0ACF418F7CFC6539AB2257EB2D5297BAFA.exit:443 SOURCE_ADDR=(Tor_internal):0 PURPOSE=DIR_FETCH ``` StreamEvent should use is_valid_port(port, allow-zero=True) due to DIR_FETCH. **Trac**: **Username**: robinson
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