mpjpeg demuxer is detected but does not work

[Carl Eugen Hoyos]

mpv issue 6964, not directly related to #5023 afaict.
A sample was provided that does work with -f mjpeg, does not work since 6dc1d5f87ced885d748f8d0c7bfbc5ff6471483f when the sample was first detected as mpjpeg.

$ ffmpeg -i ztpl.idontkn -f null -
ffmpeg version N-94961-g1d86e4b3eb Copyright (c) 2000-2019 the FFmpeg developers
  built with clang version 8.0.1 (tags/RELEASE_801/final 366581)
  configuration: --cc=clang --enable-gpl --enable-gnutls --enable-libxml2 --enable-libx264 --enable-libx265 --enable-libvpx --enable-libaom --enable-libdavs2
  libavutil      56. 35.100 / 56. 35.100
  libavcodec     58. 57.100 / 58. 57.100
  libavformat    58. 32.104 / 58. 32.104
  libavdevice    58.  9.100 / 58.  9.100
  libavfilter     7. 58.102 /  7. 58.102
  libswscale      5.  6.100 /  5.  6.100
  libswresample   3.  6.100 /  3.  6.100
  libpostproc    55.  6.100 / 55.  6.100
Input #0, mpjpeg, from 'ztpl.idontkn':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: mjpeg (Baseline), yuvj420p(pc, bt470bg/unknown/unknown), 638x478 [SAR 1:1 DAR 319:239], 25 tbr, 25 tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (native) -> wrapped_avframe (native))
Press [q] to stop, [?] for help
ztpl.idontkn: corrupt input packet in stream 0
Output #0, null, to 'pipe:':
  Metadata:
    encoder         : Lavf58.32.104
    Stream #0:0: Video: wrapped_avframe, yuvj420p, 638x478 [SAR 1:1 DAR 319:239], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc58.57.100 wrapped_avframe
frame=    1 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.04 bitrate=N/A speed=23.2x    
video:1kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

The sample contains 95 frames.

[Elon Musk]

File is invalid mpjpeg, with bad boundary signatures.