Unstable outputs ffmpeg (the second one)

[Shaohua Li]

Summary of the bug: For some inputs, ffmpeg's output file will change per run. I have reported a similar issue before (see ​https://trac.ffmpeg.org/ticket/9272). But that one is a threading issue and -threads 1 makes the issue disappear. However, this seems not to be a threading issue since with -threads 1 the output files still change per run.
Compiler: gcc11
How to reproduce: (run the following command several times, and check the size of output.mp4 in each run. )

% ffmpeg -threads 1 -y -i input.avi -f mp4 output.mp4
ffmpeg version N-104353-g2c734a8496 Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 11 (Ubuntu 11.1.0-1ubuntu1~20.04)
  configuration: --cc=/afl_diff_rand/afl_diff/compilers/gcc11-base/afl-gcc --cxx=/afl_diff_rand/afl_diff/compilers/gcc11-base/afl-g++ --ld='/afl_diff_rand/afl_diff/compilers/gcc11-base/afl-g++ -fno-sanitize=vptr -fno-sanitize=vptr -fno-sanitize=vptr -fno-sanitize=vptr -fno-sanitize=vptr -fno-sanitize=vptr -fno-sanitize=vptr -fno-sanitize=vptr -fno-sanitize=vptr -std=c++11' --extra-cflags=-I/ffmpeg/repo/gcc11-O2/ffmpeg_deps/include --extra-ldflags=-L/ffmpeg/repo/gcc11-O2/ffmpeg_deps/lib --prefix=/ffmpeg/repo/gcc11-O2/ffmpeg_deps --pkg-config-flags=--static --optflags=-O2 --enable-gpl --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libopus --enable-libtheora --enable-libvorbis --enable-nonfree --disable-shared
  libavutil      57.  7.100 / 57.  7.100
  libavcodec     59. 12.100 / 59. 12.100
  libavformat    59.  6.100 / 59.  6.100
  libavdevice    59.  0.101 / 59.  0.101
  libavfilter     8. 14.100 /  8. 14.100
  libswscale      6.  1.100 /  6.  1.100
  libswresample   4.  0.100 /  4.  0.100
  libpostproc    56.  0.100 / 56.  0.100

[Shaohua Li]

input.avi

[Elon Musk]

Input is invalid data as reported by ffmpeg.