Fix invalid line stride computation causing OOB array accesses

Fixes #125

Author: lovasoa

src/decoder.rs

@@ -785,7 +785,7 @@ fn compute_image(components: &[Component],
         let height = component.size.height as usize;
 
         let mut buffer = vec![0u8; width * height];
-        let line_stride = width * component.dct_scale;
+        let line_stride = component.block_size.width as usize * component.dct_scale;
 
         for y in 0 .. width {
             for x in 0 .. height {