Merge pull request image-rs#206 from 5225225/sub-with-overflow-coeff

Avoid integer overflow when summing coefficients

src/decoder.rs

@@ -939,19 +939,21 @@ fn refine_non_zeroes<R: Read>(reader: &mut R,
  for i in range {
  let index = UNZIGZAG[i as usize] as usize;
 
- if coefficients[index] == 0 {
+ let coefficient = &mut coefficients[index];
+
+ if *coefficient == 0 {
  if zero_run_length == 0 {
  return Ok(i);
  }
 
  zero_run_length -= 1;
  }
- else if huffman.get_bits(reader, 1)? == 1 && coefficients[index] & bit == 0 {
- if coefficients[index] > 0 {
- coefficients[index] += bit;
+ else if huffman.get_bits(reader, 1)? == 1 && *coefficient & bit == 0 {
+ if *coefficient > 0 {
+ *coefficient = coefficient.checked_add(bit).ok_or_else(|| Error::Format("Coefficient overflow".to_owned()))?;
  }
  else {
- coefficients[index] -= bit;
+ *coefficient = coefficient.checked_sub(bit).ok_or_else(|| Error::Format("Coefficient overflow".to_owned()))?;
  }
  }
  }