message: Check for violation of maxval
diff: @@ -31,6 +31,10 @@ impl<R: Read> PPMDecoder<R> {
  let height = try!(PPMDecoder::read_next_u32(&mut buf));
  let maxwhite = try!(PPMDecoder::read_next_u32(&mut buf));
  + if !(maxwhite <= u16::max_value() as u32) {
  + return Err(ImageError::FormatError("Image maxval is not less or equal to 65535".to_string()))
  + }
  +
  Ok(PPMDecoder {
  reader: buf,
  width: width,
  