webm to png sequence: wrong and insane framerate

[php4fan]

Summary of the bug:
How to reproduce:

% ffmpeg -i example.webm %d.png
$ ffmpeg -i come_in_hor.webm -vf "crop=200:247:0:24" come_in_hor/%d.png
ffmpeg version 2.8.15-0ubuntu0.16.04.1 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 5.4.0 (Ubuntu 5.4.0-6ubuntu1~16.04.10) 20160609
  configuration: --prefix=/usr --extra-version=0ubuntu0.16.04.1 --build-suffix=-ffmpeg --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --cc=cc --cxx=g++ --enable-gpl --enable-shared --disable-stripping --disable-decoder=libopenjpeg --disable-decoder=libschroedinger --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librtmp --enable-libschroedinger --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxvid --enable-libzvbi --enable-openal --enable-opengl --enable-x11grab --enable-libdc1394 --enable-libiec61883 --enable-libzmq --enable-frei0r --enable-libx264 --enable-libopencv
  libavutil      54. 31.100 / 54. 31.100
  libavcodec     56. 60.100 / 56. 60.100
  libavformat    56. 40.101 / 56. 40.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 40.101 /  5. 40.101
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  2.101 /  1.  2.101
  libpostproc    53.  3.100 / 53.  3.100
Input #0, matroska,webm, from 'come_in_hor.webm':
  Metadata:
    encoder         : Chrome
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0(eng): Video: h264 (Constrained Baseline), yuv420p, 634x348, SAR 1:1 DAR 317:174, 1k tbr, 1k tbn, 2k tbc (default)
Output #0, image2, to 'come_in_hor/%d.png':
  Metadata:
    encoder         : Lavf56.40.101
    Stream #0:0(eng): Video: png, rgb24, 200x246 [SAR 1:1 DAR 100:123], q=2-31, 200 kb/s, 1k fps, 1k tbn, 1k tbc (default)
    Metadata:
      encoder         : Lavc56.60.100 png
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> png (native))
Press [q] to stop, [?] for help
[h264 @ 0x1d9acc0] Reinit context to 640x304, pix_fmt: yuv420pN/A dup=1954 drop=0    
Input stream #0:0 frame changed from size:634x348 fmt:yuv420p to size:634x300 fmt:yuv420p
frame= 3757 fps=1165 q=-0.0 Lsize=N/A time=00:00:03.75 bitrate=N/A dup=3704 drop=0    
video:38033kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

Note the insane rerported framerate of 1165. This produced a ton of png files.

I don't know the exact frame rate of this file but it's certainly not 1165.

I suspect if I were converting to any other video format I would have the same issue: the issue seems to be that the framerate of the input file is not read correctly.