GH-98539: fix ref cycle in `_SSLProtocolTransport` after close (#98540)

Lib/asyncio/sslproto.py

@@ -107,8 +107,11 @@ def close(self):
         protocol's connection_lost() method will (eventually) called
         with None as its argument.
         """
+        if not self._closed:
             self._closed = True
             self._ssl_protocol._start_shutdown()
+        else:
+            self._ssl_protocol = None
 
     def __del__(self, _warnings=warnings):
         if not self._closed: