Returns stream_user_data for the stream *stream_id*.  The
stream_user_data is provided by `nghttp2_submit_request2()`,
`nghttp2_submit_headers()` or
`nghttp2_session_set_stream_user_data()`.  Unless it is set using
`nghttp2_session_set_stream_user_data()`, if the stream is
initiated by the remote endpoint, stream_user_data is always
``NULL``.  If the stream does not exist, this function returns
``NULL``.