bool checkreturn pb_make_string_substream(pb_istream_t *stream, pb_istream_t *substream)
{
    uint32_t size;
    if (!pb_decode_varint32(stream, &size))
        return false;
    
    *substream = *stream;
    if (substream->bytes_left < size)
        PB_RETURN_ERROR(stream, "parent stream too short");
    
    substream->bytes_left = size;
    stream->bytes_left -= size;
    return true;
}