Fix race condition when closing a channel

This commit fixes a race between a client sending a channel close
message and receiving an exit status or other channel requests
from the server. Thanks go to Wilson Conley for providing sample
code to reproduce the problem!

asyncssh/channel.py

@@ -413,7 +413,11 @@ def _service_next_request(self) -> None:
         handler = cast(_RequestHandler, getattr(self, name, None))
 
         if handler:
-            result = cast(Optional[bool], handler(packet))
+            if self._session:
+                result = cast(Optional[bool], handler(packet))
+            else:
+                # Ignore requests received after application closes the channel
+                result = True
         else:
             self.logger.debug1('Received unknown channel request: %s', request)
             result = False