Avoid infinite loop when retrying after earlier fatal error.

[anforowicz]

When StreamingDecoder reports an error, it leaves state set to None. Before this commit, calling next_frame in this state would have led to an infinite loop:

• ReadDecoder::decode_next would loop forever (!self.at_eof is true after an error) and would fail to make progress, because
• When StreamingDecoder::update saw state set to None then before this commit it wouldn't enter the next_state loop and would immediately return no progress (Ok((/* consumed bytes = */ 0, Decoded::Nothing))).

After this commit, StreamingDecoder::update checks if the state is None and treats this as an error.

Code links:

• ReadDecoder::decode_next:

  image-png/src/decoder/mod.rs

  Lines 299 to 316 in 272ae60

  	fn decode_next(&mut self, image_data: &mut Vec<u8>) -> Result<Option<Decoded>, DecodingError> {
  	while !self.at_eof {
  	let (consumed, result) = {
  	let buf = self.reader.fill_buf()?;
  	if buf.is_empty() {
  	return Err(DecodingError::IoError(ErrorKind::UnexpectedEof.into()));
  	}
  	self.decoder.update(buf, image_data)?
  	};
  	self.reader.consume(consumed);
  	match result {
  	Decoded::Nothing => (),
  	Decoded::ImageEnd => self.at_eof = true,
  	result => return Ok(Some(result)),
  	}
  	}
  	Ok(None)
  	}

• StreamingDecoder::update:

  image-png/src/decoder/stream.rs

  Lines 625 to 642 in 272ae60

  	pub fn update(
  	&mut self,
  	mut buf: &[u8],
  	image_data: &mut Vec<u8>,
  	) -> Result<(usize, Decoded), DecodingError> {
  	let len = buf.len();
  	while !buf.is_empty() && self.state.is_some() {
  	match self.next_state(buf, image_data) {
  	Ok((bytes, Decoded::Nothing)) => buf = &buf[bytes..],
  	Ok((bytes, result)) => {
  	buf = &buf[bytes..];
  	return Ok((len - buf.len(), result));
  	}
  	Err(err) => return Err(err),
  	}
  	}
  	Ok((len - buf.len(), Decoded::Nothing))
  	}