函数 from_buf 

pub fn from_buf(
    rd: &mut dyn BufRead,
) -> Result<Option<(SectionKind, Vec<u8>)>, Error>

从 rd 中提取并解码下一个所支持的 PEM section。

• Ok(None) is returned if there is no PEM section read from rd.
• Underlying IO errors produce a Err(...)
• Otherwise each decoded section is returned with a Ok(Some(...))