Returns pointer to :type:`nghttp2_stream` object denoted by
*stream_id*.  If stream was not found, returns NULL.

Returns imaginary root stream (see
`nghttp2_session_get_root_stream()`) if 0 is given in *stream_id*.

Unless *stream_id* == 0, the returned pointer is valid until next
call of `nghttp2_session_send()`, `nghttp2_session_mem_send2()`,
`nghttp2_session_recv()`, and `nghttp2_session_mem_recv2()`.