dnxhdenc: fix invalid reads in dnxhd_mb_var_thread().

[ffmpeg] / tests / ref / vsynth / vsynth1-mjpeg

b3ff9a5a9699ceddfee9abbf1b06bb00 *tests/data/fate/vsynth1-mjpeg.avi
1516128 tests/data/fate/vsynth1-mjpeg.avi
c6ae81b5b896e4d05ff584311aebdb18 *tests/data/fate/vsynth1-mjpeg.out.rawvideo
stddev:    7.87 PSNR: 30.21 MAXDIFF:   63 bytes:  7603200/  7603200