Handle outbound messages to non-existent clients more gracefully.

```
2014-06-05 17:40:44+0000 [VumiRedis,client] Unhandled Error
        Traceback (most recent call last):
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 577, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/var/praekelt/vumi-go/ve/src/vumi/vumi/transports/base.py", line 178, in processor
            d = maybeDeferred(handler, message)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 139, in maybeDeferred
            result = f(*args, **kw)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1237, in unwindGenerator
            return _inlineCallbacks(None, gen, Deferred())
        --- <exception caught here> ---
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1099, in _inlineCallbacks
            result = g.send(result)
          File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/vxfreeswitch/voice.py", line 299, in handle_outbound_message
            client.set_input_type(None)
        exceptions.AttributeError: 'NoneType' object has no attribute 'set_input_type'
```