Main image data in DNGs is usually comprised of tiles, each of which is a Huffman-encoded lossless JPEG.

Tested for ljpeg regressions with:
`ffmpeg -f lavfi -i testsrc=d=1 -vcodec ljpeg test.avi`
`ffmpeg test.avi out.avi`
The modified code in ljpeg_decode_rgb_scan runs without issues.
Signed-off-by: Nick Renieris <velocityra@gmail.com>