skip_alpha breaks ffv1 decoding

[Carl Eugen Hoyos]

$ ffmpeg -f lavfi -i testsrc -pix_fmt yuva420p -vcodec ffv1 -t 1 out.avi

$ ffmpeg -skip_alpha 1 -i out.avi -f null -
ffmpeg version N-69945-g2280552 Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      54. 18.100 / 54. 18.100
  libavcodec     56. 22.100 / 56. 22.100
  libavformat    56. 22.100 / 56. 22.100
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 11.100 /  5. 11.100
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, avi, from 'out.avi':
  Metadata:
    encoder         : Lavf56.22.100
  Duration: 00:00:01.00, start: 0.000000, bitrate: 871 kb/s
    Stream #0:0: Video: ffv1 (FFV1 / 0x31564646), yuv420p, 320x240, 855 kb/s, SAR 1:1 DAR 4:3, 25 fps, 25 tbr, 25 tbn, 25 tbc
Output #0, null, to 'pipe:':
  Metadata:
    encoder         : Lavf56.22.100
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 320x240 [SAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc56.22.100 rawvideo
Stream mapping:
  Stream #0:0 -> #0:0 (ffv1 (native) -> rawvideo (native))
Press [q] to stop, [?] for help
[null @ 0x3584100] Encoder did not produce proper pts, making some up.
[ffv1 @ 0x3575560] Invalid change of global parameters
[ffv1 @ 0x3576360] Cannot decode non-keyframe without valid keyframe
[ffv1 @ 0x357bfc0] Cannot decode non-keyframe without valid keyframe
[ffv1 @ 0x357e8e0] Cannot decode non-keyframe without valid keyframe
[ffv1 @ 0x3564100] Cannot decode non-keyframe without valid keyframe
[ffv1 @ 0x3566f20] Cannot decode non-keyframe without valid keyframe
[ffv1 @ 0x3589cc0] Cannot decode non-keyframe without valid keyframe
Error while decoding stream #0:0: Invalid data found when processing input
[ffv1 @ 0x3570040] Cannot decode non-keyframe without valid keyframe
Error while decoding stream #0:0: Invalid data found when processing input
[ffv1 @ 0x3572960] Cannot decode non-keyframe without valid keyframe
Error while decoding stream #0:0: Invalid data found when processing input
[ffv1 @ 0x3575560] Cannot decode non-keyframe without valid keyframe
Error while decoding stream #0:0: Invalid data found when processing input
[ffv1 @ 0x3576360] Cannot decode non-keyframe without valid keyframe
Error while decoding stream #0:0: Invalid data found when processing input
[ffv1 @ 0x357bfc0] Cannot decode non-keyframe without valid keyframe
Error while decoding stream #0:0: Invalid data found when processing input
[ffv1 @ 0x357e8e0] Invalid change of global parameters
frame=   12 fps=0.0 q=0.0 Lsize=N/A time=00:00:00.48 bitrate=N/A
video:1kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

[Michael Niedermayer]

Fixed in cf52e6d012d035b82080e6cc045378f900627870