mjpeg: tweak rstn handling for progressive mode

[ami_stuff]

There seems to be still some problems with rstn handling, I have uploaded some more samples:

​http://www.datafilehost.com/download-9735e969.html

C:\>ffmpeg -i rstn5.jpg out.bmp
ffmpeg version N-48228-gceee440 Copyright (c) 2000-2012 the FFmpeg developers
  built on Dec 27 2012 02:22:24 with gcc 4.5.0 (GCC) 20100414 (Fedora MinGW 4.5.
0-1.fc14)
  configuration: --prefix=/var/www/users/research/ffmpeg/snapshots/build --arch=
x86 --target-os=mingw32 --cross-prefix=i686-pc-mingw32- --cc='ccache i686-pc-min
gw32-gcc' --enable-w32threads --enable-memalign-hack --enable-runtime-cpudetect
--enable-cross-compile --enable-static --disable-shared --extra-libs='-lws2_32 -
lwinmm -lpthread' --extra-cflags='--static -I/var/www/users/research/ffmpeg/snap
shots/build/include' --extra-ldflags='-static -L/var/www/users/research/ffmpeg/s
napshots/build/lib' --enable-bzlib --enable-zlib --enable-gpl --enable-version3
--enable-nonfree --enable-libx264 --enable-libspeex --enable-libtheora --enable-
libvorbis --enable-libfaac --enable-libxvid --enable-libopencore-amrnb --enable-
libopencore-amrwb --enable-libmp3lame --enable-libvpx --disable-decoder=libvpx
  libavutil      52. 12.100 / 52. 12.100
  libavcodec     54. 81.100 / 54. 81.100
  libavformat    54. 50.102 / 54. 50.102
  libavdevice    54.  3.102 / 54.  3.102
  libavfilter     3. 30.101 /  3. 30.101
  libswscale      2.  1.103 /  2.  1.103
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
[mjpeg @ 0x1eff5e0] error count: 268435456
[mjpeg @ 0x1eff5e0] error y=5 x=0
[mjpeg @ 0x1eff5e0] error count: 268435456
[mjpeg @ 0x1eff5e0] error y=9 x=126
[mjpeg @ 0x1eff5e0] error count: 268435456
[mjpeg @ 0x1eff5e0] error y=5 x=0
Input #0, image2, from 'rstn5.jpg':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: mjpeg, yuvj444p, 1024x768 [SAR 96:96 DAR 4:3], 25 tbr, 2
5 tbn, 25 tbc
Output #0, image2, to 'out.bmp':
  Metadata:
    encoder         : Lavf54.50.102
    Stream #0:0: Video: bmp, bgr24, 1024x768 [SAR 1:1 DAR 4:3], q=2-31, 200 kb/s
, 90k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg -> bmp)
Press [q] to stop, [?] for help
[mjpeg @ 0x1eff5e0] error count: 268435456
[mjpeg @ 0x1eff5e0] error y=5 x=0
[mjpeg @ 0x1eff5e0] error count: 268435456
[mjpeg @ 0x1eff5e0] error y=9 x=126
[mjpeg @ 0x1eff5e0] error count: 268435456
[mjpeg @ 0x1eff5e0] error y=5 x=0
frame=    1 fps=0.0 q=0.0 Lsize=       0kB time=00:00:00.04 bitrate=   0.0kbits/
s
video:2304kB audio:0kB subtitle:0 global headers:0kB muxing overhead -100.000000
%