ljpeg: support/reject non-interleaved files

[ami_stuff]

C:\>ffmpeg -i nint.jpg
ffmpeg version N-36193-gf514695, Copyright (c) 2000-2011 the FFmpeg developers
  built on Dec 26 2011 17:50:37 with gcc 4.6.2
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-ru
ntime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libope
ncore-amrnb --enable-libopencore-amrwb --enable-libfreetype --enable-libgsm --en
able-libmp3lame --enable-libopenjpeg --enable-librtmp --enable-libschroedinger -
-enable-libspeex --enable-libtheora --enable-libvo-aacenc --enable-libvo-amrwben
c --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-
libxvid --enable-zlib
  libavutil      51. 33.100 / 51. 33.100
  libavcodec     53. 48.100 / 53. 48.100
  libavformat    53. 28.100 / 53. 28.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 54.100 /  2. 54.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  5.100 /  0.  5.100
  libpostproc    51.  2.100 / 51.  2.100
[mjpeg @ 02645B60] mjpeg_decode_dc: bad vlc: 0:0 (020E9A68)
    Last message repeated 2 times
Input #0, image2, from 'nint.jpg':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: mjpeg, bgr24, 640x480, 25 tbr, 25 tbn, 25 tbc
At least one output file must be specified

[Michael Niedermayer]

locally fixed