qdrw: fix decoding of 8bpp sample

[ami_stuff]

decodes ok with photoshop/quicktime/imagemagick

​https://samples.ffmpeg.org/image-samples/tess19.pct

C:\>ffmpeg -i C:\tess19.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 @ 0237d8a0] Warning: not compiled with thread support, using thread emula
tion
[qdraw @ 0237d8a0] Palette index out of range: 2048
    Last message repeated 255 times
Input #0, qdraw_pipe, from 'C:\tess19.pct':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: qdraw, pal8, 574x419, 25 tbr, 25 tbn, 25 tbc
File 'out.bmp' already exists. Overwrite ? [y/N] y
[qdraw @ 02750400] Warning: not compiled with thread support, using thread emula
tion
[bmp @ 02376c40] 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, 574x419, 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
[qdraw @ 02750400] Palette index out of range: 2048
    Last message repeated 255 times
frame=    1 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.04 bitrate=N/A speed=0.256x

video:237kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing o
verhead: unknown

[Carl Eugen Hoyos]

Patch sent.

[Carl Eugen Hoyos]

Should be fixed in 6dc79637f34f4a5a76f4bf9e66b78a27d156b1df