jpg 4:4:4 encoder does not work correctly with -flags gray

[cehoyos]

The output file shows artefacts.

$ ffmpeg -i tests/lena.pnm -flags gray -pix_fmt yuvj444p out.jpg
ffmpeg version N-49005-g44d5a28 Copyright (c) 2000-2013 the FFmpeg developers
  built on Jan 18 2013 20:45:11 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl --disable-indev=jack
  libavutil      52. 15.100 / 52. 15.100
  libavcodec     54. 89.100 / 54. 89.100
  libavformat    54. 61.100 / 54. 61.100
  libavdevice    54.  3.102 / 54.  3.102
  libavfilter     3. 32.100 /  3. 32.100
  libswscale      2.  1.103 /  2.  1.103
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
Input #0, image2, from 'tests/lena.pnm':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: ppm, rgb24, 256x256, 25 tbr, 25 tbn, 25 tbc
Output #0, image2, to 'out.jpg':
  Metadata:
    encoder         : Lavf54.61.100
    Stream #0:0: Video: mjpeg, yuvj444p, 256x256, q=2-31, 200 kb/s, 90k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (ppm -> mjpeg)
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=0.0 Lsize=N/A time=00:00:00.04 bitrate=N/A
video:13kB audio:0kB subtitle:0 global headers:0kB muxing overhead -100.170226%