Handle Malformed URIs Bug

Consider adding `if not uri: return False` or equivalent to handle missing URIs.

Otherwise:
```
2014-11-21 11:39:16,740 - tornado.application - ERROR - Exception in callback None
Traceback (most recent call last):
  File "/home/awbreys/OpenBazaar/env/local/lib/python2.7/site-packages/tornado/ioloop.py", line 837, in start
    handler_func(fd_obj, events)
  File "/home/awbreys/OpenBazaar/env/local/lib/python2.7/site-packages/tornado/stack_context.py", line 275, in null_wrapper
    return fn(*args, **kwargs)
  File "/home/awbreys/OpenBazaar/env/local/lib/python2.7/site-packages/zmq/eventloop/zmqstream.py", line 433, in _handle_events
    self._handle_recv()
  File "/home/awbreys/OpenBazaar/env/local/lib/python2.7/site-packages/zmq/eventloop/zmqstream.py", line 465, in _handle_recv
    self._run_callback(callback, msg)
  File "/home/awbreys/OpenBazaar/env/local/lib/python2.7/site-packages/zmq/eventloop/zmqstream.py", line 407, in _run_callback
    callback(*args, **kwargs)
  File "/home/awbreys/OpenBazaar/env/local/lib/python2.7/site-packages/tornado/stack_context.py", line 275, in null_wrapper
    return fn(*args, **kwargs)
  File "node/connection.py", line 270, in handle_recv
    self._on_raw_message(msg)
  File "node/connection.py", line 356, in _on_raw_message
    self._data_cb(message)
  File "node/transport.py", line 477, in _on_message
    if not network_util.valid_uri(uri):
  File "node/network_util.py", line 104, in valid_uri
    require_authority=True)
  File "/home/awbreys/OpenBazaar/env/local/lib/python2.7/site-packages/rfc3986/api.py", line 62, in is_valid_uri
    return URIReference.from_string(uri, encoding).is_valid(**kwargs)
  File "/home/awbreys/OpenBazaar/env/local/lib/python2.7/site-packages/rfc3986/uri.py", line 67, in from_string
    split_uri = URI_MATCHER.match(uri_string).groupdict()
TypeError: expected string or buffer
```