Break from handle_write() if nothing was sent

asyncore will return 0 if the socket raised EWOULDBLOCK on the send()
call

cassandra/io/asyncorereactor.py

@@ -246,6 +246,8 @@ class AsyncoreConnection(Connection, asyncore.dispatcher):
                 if sent < len(next_msg):
                     with self.deque_lock:
                         self.deque.appendleft(next_msg[sent:])
+                    if sent == 0:
+                        return
 
     def handle_read(self):
         try: