title: Video merge using ffmpeg issue
body: <p>pretty basic node script im using to merge 2 mp4 files together, currently have an error:</p>
  
  <p>Error ffmpeg exited with code 1: Error reinitializing filters!
  Failed to inject frame into filter network: Invalid argument
  Error while processing the decoded data for stream #1:0
  Conversion failed!</p>
  
  <p>if i merge 2 of the same video together it works but when i use 2 different videos it throws that error!. i used the first code snippet to change the size and aspect ration but still no luck! I have spent hours on end search for this fix. how on earth do you merge 2 different videos together. </p>
  
  <p>i've seen people comment that:
  1. sizing is different
  2. aspect is out of whack
  3. you can use Unsafe = 1 ???? 
  4. maybe different frame rate on the files?</p>
  
  <p>any help would be awesome! cheers!</p>
  
  <pre><code>var fluent_ffmpeg = require("fluent-ffmpeg");
  var fs = require('fs');
  var ffmpeg = require('fluent-ffmpeg');
  
  
  //ATTEMPT TO RESIZE VIDEO FILES
  //-------------------------------------------------------
  var command = ffmpeg();
  var command2 = ffmpeg();
  
  var command = ffmpeg('./save/pokemon/name0.mp4')
    .videoCodec('libx264')
    .format('mp4');
  
  var command2 = ffmpeg('./save/pokemon/name1.mp4')
    .videoCodec('libx264')
    .format('mp4');
  
  command.clone()
    .size('640x?').aspect('4:3').autopad()
    .save('./save/name0.mp4')
  
  command2.clone()
    .size('640x?').aspect('4:3').autopad()
    .save('./save/name1.mp4');
  //-------------------------------------------------------
  
  </code></pre>
  
  <pre><code>//MERGE VIDEOS TOGETHER
  //-------------------------------------------------------
  async function test()
  {
      var mergedVideo = fluent_ffmpeg();
  
      var videoNames  = ['./save/name0.mp4','./save/name1.mp4']
  
      videoNames.forEach(function(videoName)
      {
          mergedVideo = mergedVideo.addInput(videoName);
      });
  
      mergedVideo.mergeToFile('./save/mergedVideo.avi', './tmp/').on('error', function(err) 
      {
          console.log('Error ' + err.message);
  
      }).on('end', function() 
      {
          console.log('Finished!');
      });
  }
  //-------------------------------------------------------
  </code></pre>
  
  <pre><code>C:\gif&gt;ffmpeg -i ./save/pokemon/name0.mp4 -i ./save/pokemon/name1.mp4
  ffmpeg version 4.2.2 Copyright (c) 2000-2019 the FFmpeg developers
    built with gcc 9.2.1 (GCC) 20200122
    configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt
    libavutil      56. 31.100 / 56. 31.100
    libavcodec     58. 54.100 / 58. 54.100
    libavformat    58. 29.100 / 58. 29.100
    libavdevice    58.  8.100 / 58.  8.100
    libavfilter     7. 57.100 /  7. 57.100
    libswscale      5.  5.100 /  5.  5.100
    libswresample   3.  5.100 /  3.  5.100
    libpostproc    55.  5.100 / 55.  5.100
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from './save/pokemon/name0.mp4':
    Metadata:
      major_brand     : isom
      minor_version   : 512
      compatible_brands: isomiso2avc1mp41
      encoder         : Lavf58.26.101
    Duration: 00:00:01.14, start: 0.000000, bitrate: 1104 kb/s
      Stream #0:0(und): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p, 480x334 [SAR 1:1 DAR 240:167], 1098 kb/s, 33.33 fps, 33.33 tbr, 333333 tbn, 66.67 tbc (default)
      Metadata:
        handler_name    : VideoHandler
  Input #1, mov,mp4,m4a,3gp,3g2,mj2, from './save/pokemon/name1.mp4':
    Metadata:
      major_brand     : isom
      minor_version   : 512
      compatible_brands: isomiso2avc1mp41
      encoder         : Lavf58.26.101
      comment         : PhotoScape
    Duration: 00:00:01.80, start: 0.000000, bitrate: 1157 kb/s
      Stream #1:0(und): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p, 480x358 [SAR 1:1 DAR 240:179], 1153 kb/s, 6.67 fps, 6.67 tbr, 666667 tbn, 13.33 tbc (default)
      Metadata:
        handler_name    : VideoHandler
  At least one output file must be specified
  </code></pre>