errors when the connection is closed as we process it

xpra/server/server_core.py

@@ -2026,6 +2026,9 @@ def auth_failed(msg):
             if v:
                 remote[key] = v
         conn = proto._conn
+        if not conn or proto.is_closed():
+            authlog(f"connection {proto} is already closed")
+            return
         #authenticator:
         if not proto.authenticators:
             socktype = conn.socktype_wrapped