USSD transports unhappy after a disconnect.

```
2014-02-01 10:19:08+0000 [Uninitialized] Unhandled Error
        Traceback (most recent call last):
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/python/log.py", line 88, in callWithLogger
            return callWithContext({"system": lp}, func, *args, **kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/python/log.py", line 73, in callWithContext
            return context.call({ILogContext: newCtx}, func, *args, **kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/python/context.py", line 118, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/python/context.py", line 81, in callWithContext
            return func(*args,**kw)
        --- <exception caught here> ---
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/posixbase.py", line 619, in _doReadOrWrite
            why = selectable.doWrite()
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/tcp.py", line 593, in doConnect
            self._connectDone()
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/tcp.py", line 612, in _connectDone
            self.protocol.makeConnection(self)
        exceptions.AttributeError: 'NoneType' object has no attribute 'makeConnection'

2014-02-01 10:19:08+0000 [Uninitialized] Unhandled Error
        Traceback (most recent call last):
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/application/app.py", line 313, in runReactorWithLogging
            reactor.run()
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/base.py", line 1192, in run
            self.mainLoop()
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/base.py", line 1204, in mainLoop
            self.doIteration(t)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/epollreactor.py", line 396, in doPoll
            log.callWithLogger(selectable, _drdw, selectable, fd, event)
        --- <exception caught here> ---
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/python/log.py", line 88, in callWithLogger
            return callWithContext({"system": lp}, func, *args, **kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/python/log.py", line 73, in callWithContext
            return context.call({ILogContext: newCtx}, func, *args, **kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/python/context.py", line 118, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/python/context.py", line 81, in callWithContext
            return func(*args,**kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/posixbase.py", line 627, in _doReadOrWrite
            self._disconnectSelectable(selectable, why, inRead)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/posixbase.py", line 263, in _disconnectSelectable
            selectable.connectionLost(failure.Failure(why))
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/tcp.py", line 485, in connectionLost
            self._commonConnection.connectionLost(self, reason)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/tcp.py", line 299, in connectionLost
            protocol.connectionLost(reason)
            exceptions.AttributeError: 'NoneType' object has no attribute 'connectionLost'

```