message: Correctly constrain armor begin marker
diff: @@ -152,7 +152,18 @@ impl<R: Read> Read for ArmoredReader<R> {
  self.line_buf.clear();
  self.inner.read_line(&mut self.line_buf)?;
  + // The first line of armor is the armor marker followed by either
  + // CRLF or LF.
  let is_armored = self.line_buf.starts_with(ARMORED_BEGIN_MARKER);
  + if is_armored {
  + let remainder = &self.line_buf.as_bytes()[ARMORED_BEGIN_MARKER.len()..];
  + if !(remainder == b"\r\n" || remainder == b"\n") {
  + return Err(io::Error::new(
  + io::ErrorKind::InvalidData,
  + "invalid armor begin marker",
  + ));
  + }
  + }
  self.is_armored = Some(is_armored);
  }
  Some(false) => {
  