Fix error when reset

If `Mplex` is cleanup first, `MplexStream.reset` possibly fails because
`Mplex.streams` is set to `None` in `cleanup`.

libp2p/stream_muxer/mplex/mplex_stream.py

@@ -204,6 +204,10 @@ class MplexStream(IMuxedStream):
             self.event_remote_closed.set()
 
         async with self.mplex_conn.streams_lock:
+            if (
+                self.mplex_conn.streams is not None
+                and self.stream_id in self.mplex_conn.streams
+            ):
                 del self.mplex_conn.streams[self.stream_id]
 
     # TODO deadline not in use