Set websocket.terminal to None when the child exits

This fixes an error I was seeing when exiting a terminal which is open
in multiple tabs at the same time. The code path to resize the terminal
to the smallest client window size was taken, which tries to communicate
with the (now closed) terminal.

terminado/websocket.py

@@ -100,3 +100,4 @@ def on_pty_died(self):
         """
         self.send_json_message(['disconnect', 1])
         self.close()
+        self.terminal = None