socket: Initialize client_id to None by default.

This fixes an exception where client_id was never set in an error code
path.  It shouldn't be needed, but I think this makes the code clearer
and this will help in debugging the actual problem.

Related to #753.

zerver/lib/socket.py

@@ -72,6 +72,8 @@ class CloseErrorInfo(object):
         self.err_msg = err_msg
 
 class SocketConnection(sockjs.tornado.SockJSConnection):
+    client_id = None # type: str
+
     def on_open(self, info):
         log_data = dict(extra='[transport=%s]' % (self.session.transport_name,))
         record_request_start_data(log_data)