jpeg: crash with lowres and weird subsamples

[ami_stuff]

Starting program: d:\mingw\msys\1.0\ffmpeg-head-4246032\ffmpeg_G.exe -vlowres 1
-i 422_22_21_12.jpg
[New Thread 2676.0xb2c]
ffmpeg version 0.9.1.git-4246032 Copyright (c) 2000-2012 the FFmpeg developers
  built on Mar 22 2012 16:12:00 with gcc 4.6.1
  configuration: --disable-ffprobe
  libavutil      51. 44.100 / 51. 44.100
  libavcodec     54. 12.100 / 54. 12.100
  libavformat    54.  2.100 / 54.  2.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 65.102 /  2. 65.102
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  7.100 /  0.  7.100

Program received signal SIGSEGV, Segmentation fault.
0x005ddc41 in ff_mjpeg_decode_frame (avctx=0x397a2e0, data=0x22f6e8,
    data_size=0x22f898, avpkt=0x22f658) at libavcodec/mjpegdec.c:1743
1743                    line[index] = (line[index / 2] + line[(index + 1) / 2])
>> 1;
(gdb) bt
#0  0x005ddc41 in ff_mjpeg_decode_frame (avctx=0x397a2e0, data=0x22f6e8,
    data_size=0x22f898, avpkt=0x22f658) at libavcodec/mjpegdec.c:1743
#1  0x004fb828 in avcodec_decode_video2 (avctx=0x397a2e0, picture=0x22f6e8,
    got_picture_ptr=0x22f898, avpkt=0x22f850) at libavcodec/utils.c:1358
#2  0x00434f17 in try_decode_frame (st=0x3981120, avpkt=<optimized out>,
    options=<optimized out>) at libavformat/utils.c:2263
#3  0x0043c777 in avformat_find_stream_info (ic=0x3972340, options=0x3981440)
    at libavformat/utils.c:2568
#4  0x0040d985 in opt_input_file (o=0x22fdb8, opt=0x3980e17 "i",
    filename=<optimized out>) at ffmpeg.c:3735
#5  0x00411d17 in parse_option (optctx=0x22fdb8, opt=<optimized out>,
    arg=0x3980e19 "422_22_21_12.jpg", options=0xafc6c0) at cmdutils.c:300
#6  0x00411f7a in parse_options (optctx=0x22fdb8, argc=5,
    argv=<optimized out>, options=0xafc6c0,
    parse_arg_function=0x40ee28 <opt_output_file>) at cmdutils.c:333
#7  0x00a9fb4d in main (argc=5, argv=0x3980d80) at ffmpeg.c:5176

[Carl Eugen Hoyos]

Patch sent.

[Carl Eugen Hoyos]

Patch committed.