artiq_devtool: detect a race condition during connect.

artiq/frontend/artiq_devtool.py

@@ -116,6 +116,10 @@ def main():
                     local_stream, peer_addr = listener.accept()
                     logger.info("Accepting %s:%s and opening SSH channel to %s:%s",
                                 *peer_addr, args.ip, port)
+                    if get_ssh().get_transport() is None:
+                        logger.error("Trying to open a channel before the transport is ready!")
+                        continue
+
                     remote_stream = get_ssh().get_transport() \
                         .open_channel('direct-tcpip', (args.ip, port), peer_addr)
                     while True: