qdrw: fix decoding 2bpp

[ami_stuff]

​https://trac.ffmpeg.org/raw-attachment/ticket/6173/2bit_777.pct

C:\>ffmpeg -i C:\2bit_777.pct out.bmp
ffmpeg version 3.2.git Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.6.2 (GCC)
  configuration: --disable-pthreads --disable-ffprobe --enable-gpl
  libavutil      55. 47.100 / 55. 47.100
  libavcodec     57. 81.100 / 57. 81.100
  libavformat    57. 66.102 / 57. 66.102
  libavdevice    57.  2.100 / 57.  2.100
  libavfilter     6. 74.100 /  6. 74.100
  libswscale      4.  3.101 /  4.  3.101
  libswresample   2.  4.100 /  2.  4.100
  libpostproc    54.  2.100 / 54.  2.100
[qdraw @ 0237d8c0] Warning: not compiled with thread support, using thread emula
tion
Input #0, qdraw_pipe, from 'C:\2bit_777.pct':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: qdraw, pal8, 777x777, 25 tbr, 25 tbn, 25 tbc
[qdraw @ 027199c0] Warning: not compiled with thread support, using thread emula
tion
[bmp @ 02377d00] Warning: not compiled with thread support, using thread emulati
on
Output #0, image2, to 'out.bmp':
  Metadata:
    encoder         : Lavf57.66.102
    Stream #0:0: Video: bmp, pal8, 777x777, q=2-31, 200 kb/s, 25 fps, 25 tbn, 25
 tbc
    Metadata:
      encoder         : Lavc57.81.100 bmp
Stream mapping:
  Stream #0:0 -> #0:0 (qdraw (native) -> bmp (native))
Press [q] to stop, [?] for help
Error while decoding stream #0:0: Invalid data found when processing input
frame=    0 fps=0.0 q=0.0 Lsize=N/A time=00:00:00.00 bitrate=N/A speed=N/A
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing ove
rhead: unknown
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters i
f used)
Conversion failed!

[Elon Musk]

Fixed in 26a7d6a301b9b6c67153c87d42db145cdc0e57cf.