Fix _ParseConnect()

[gryphius]

_ParseConnect() crashes if the client connects through a unix domain socket or if the protocol is "unknown" which I've actually seen happening a few times.

DEBUG:ppymilter: <<< Clocalhost=00U
ERROR:ppymilter:uncaptured python exception, closing channel <ppymilter.ppymilterserver.ConnectionHandler connected 127.0.0.1:57708 at 0x7ff4498f1998> (<class 'struct.error'>:unpack requires a string argument of length 2 [/usr/lib/python2.7/asyncore.py|read|83] [/usr/lib/python2.7/asyncore.py|handle_read_event|449] [/usr/lib/python2.7/asynchat.py|handle_read|147] [build/bdist.linux-x86_64/egg/ppymilter/ppymilterserver.py|read_milter_data|171] [build/bdist.linux-x86_64/egg/ppymilter/ppymilterbase.py|Dispatch|212] [build/bdist.linux-x86_64/egg/ppymilter/ppymilterbase.py|_ParseConnect|281])

This patch tries to handle all possible address family types from libmilter/mfdef.h