incorrect frame readout after seek

[hxuanyu]

This issue can be reproduced on this file ​https://www.dropbox.com/s/8oru7o1dtf24x22/clipcanvas_14348_offline.mp4?dl=0

The command to reproduce this issue is

ffplay clipcanvas_14348_offline.mp4 -ss 00:00:04
ffplay version N-89901-ga026a3efae Copyright (c) 2003-2018 the FFmpeg developers
  built with gcc 7.2.0 (GCC)

if run without -ss, then everything is OK

[Carl Eugen Hoyos]

For future tickets:
Do not report issues against ffplay if they are reproducible with ffmpeg.
Always provide the command line you tested together with the complete, uncut console output.

Originally broken in 9e5ef1c5c37208326c59d642e2dc7afd3f10b09b (needs -flags2 +showall), partly fixed in e3d7a3978b857e32b32575ff78ecc7d67a18687e (related to ticket #3186).

$ ffmpeg -ss 5 -i clipcanvas_14348_offline.mp4 -qscale 2 -an out.avi
ffmpeg version N-89925-gd4967c0 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 6.3.0 (GCC)
  configuration: --enable-gpl
  libavutil      56.  7.100 / 56.  7.100
  libavcodec     58.  9.100 / 58.  9.100
  libavformat    58.  7.100 / 58.  7.100
  libavdevice    58.  0.101 / 58.  0.101
  libavfilter     7. 11.101 /  7. 11.101
  libswscale      5.  0.101 /  5.  0.101
  libswresample   3.  0.101 /  3.  0.101
  libpostproc    55.  0.100 / 55.  0.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'clipcanvas_14348_offline.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 1
    compatible_brands: isommp41avc1
    creation_time   : 2009-02-19T18:49:05.000000Z
  Duration: 00:00:08.04, start: 0.000000, bitrate: 3129 kb/s
    Stream #0:0(eng): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv), 1024x576 [SAR 1:1 DAR 16:9], 3009 kb/s, 25 fps, 25 tbr, 600 tbn, 50 tbc (default)
    Metadata:
      creation_time   : 2009-02-19T18:49:05.000000Z
      handler_name    : ?Apple Video Media Handler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
    Metadata:
      creation_time   : 2009-02-19T18:49:05.000000Z
      handler_name    : ?Apple Sound Media Handler
    Stream #0:2(eng): Data: none (mp4s / 0x7334706D), 0 kb/s (default)
    Metadata:
      creation_time   : 2009-02-19T18:49:05.000000Z
      handler_name    : ?
    Stream #0:3(eng): Data: none (mp4s / 0x7334706D), 0 kb/s (default)
    Metadata:
      creation_time   : 2009-02-19T18:49:05.000000Z
Please use -q:a or -q:v, -qscale is ambiguous
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> mpeg4 (native))
Press [q] to stop, [?] for help
Output #0, avi, to 'out.avi':
  Metadata:
    major_brand     : mp42
    minor_version   : 1
    compatible_brands: isommp41avc1
    ISFT            : Lavf58.7.100
    Stream #0:0(eng): Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 1024x576 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc (default)
    Metadata:
      creation_time   : 2009-02-19T18:49:05.000000Z
      handler_name    : ?Apple Video Media Handler
      encoder         : Lavc58.9.100 mpeg4
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
frame=   76 fps=0.0 q=2.0 Lsize=    2612kB time=00:00:03.04 bitrate=7037.8kbits/s speed=20.4x
video:2604kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.285629%

The output file looks very broken.