Lagarith decoder doesn't account null frames

[zxc]

Summary of the bug: if the video is encoded with enabled null frames, they aren't decoded.
ffmpeg correctly shows the duration (5s) of the attached file but decodes only 1 frame instead of 150.

How to reproduce:

% ffmpeg -i "'07 - creditos.avi" -vf showinfo -f null -
ffmpeg version N-109758-gbdc76f467f Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 12.2.1 (GCC) 20221004
  configuration: --enable-libzimg --enable-avisynth --enable-libbluray --extra-cflags='-march=core-avx2 -mtune=native -pipe -s -O2' --disable-debug --enable-gpl --cross-prefix=x86_64-w64-mingw32- --target-os=mingw32 --arch=x86_64 --pkg-config-flags=--static --enable-zlib --disable-pthreads --enable-w32threads --enable-libfdk-aac --enable-libsoxr --enable-nonfree
  libavutil      57. 44.100 / 57. 44.100
  libavcodec     59. 63.100 / 59. 63.100
  libavformat    59. 38.100 / 59. 38.100
  libavdevice    59.  8.101 / 59.  8.101
  libavfilter     8. 56.100 /  8. 56.100
  libswscale      6.  8.112 /  6.  8.112
  libswresample   4.  9.100 /  4.  9.100
  libpostproc    56.  7.100 / 56.  7.100
Input #0, avi, from 'C:\Users\xyz\Downloads\07 - creditos.avi':
  Duration: 00:00:05.00, start: 0.000000, bitrate: 154 kb/s
  Stream #0:0: Video: lagarith (LAGS / 0x5347414C), yuv420p, 1920x1080, 30 fps, 30 tbr, 30 tbn
Stream mapping:
  Stream #0:0 -> #0:0 (lagarith (native) -> wrapped_avframe (native))
Press [q] to stop, [?] for help
[Parsed_showinfo_0 @ 0000016f04945300] config in time_base: 1/30, frame_rate: 30/1
[Parsed_showinfo_0 @ 0000016f04945300] config out time_base: 0/0, frame_rate: 0/0
[Parsed_showinfo_0 @ 0000016f04945300] n:   0 pts:      0 pts_time:0       duration:      1 duration_time:0.0333333 pos:     8212 fmt:yuv420p sar:0/1 s:1920x1080 i:P iskey:1 type:I checksum:0135A388 plane_checksum:[A48AA81F 9E571001 1ADBEB59] mean:[230 130 127] stdev:[27.3 8.0 8.0]
[Parsed_showinfo_0 @ 0000016f04945300] color_range:unknown color_space:unknown color_primaries:unknown color_trc:unknown
Output #0, null, to 'pipe:':
  Metadata:
    encoder         : Lavf59.38.100
  Stream #0:0: Video: wrapped_avframe, yuv420p(progressive), 1920x1080, q=2-31, 200 kb/s, 30 fps, 30 tbn
    Metadata:
      encoder         : Lavc59.63.100 wrapped_avframe
frame=    0 fps=0.0 q=-0.0 size=       0kB time=00:00:00.00 bitrate=N/A speed=   0x    
frame=    1 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.00 bitrate=N/A speed=   0x    
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

[Elon Musk]

This is internal decision. Complain on devel mailing list. Not only lagarith is affected but many other codecs too.