fps filter can drop only frame

[Carl Eugen Hoyos]

​http://ffmpeg.org/pipermail/ffmpeg-user/2018-August/040928.html
A user provided a multimedia file containing only one video frame (but sound). The fps filter currently drops a single frame in a video stream since eea64ef4cfb593cbe28465f45e6bd4c41a79cae1, related to ticket #2674.

$ ffmpeg -i video_only_1frame.mp4 -vf fps=1 out.mov
ffmpeg version N-91626-gf631c32 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 6.4.0 (GCC)
  configuration: --enable-gpl --enable-gnutls --enable-libxml2
  libavutil      56. 19.100 / 56. 19.100
  libavcodec     58. 23.100 / 58. 23.100
  libavformat    58. 17.101 / 58. 17.101
  libavdevice    58.  4.101 / 58.  4.101
  libavfilter     7. 26.100 /  7. 26.100
  libswscale      5.  2.100 /  5.  2.100
  libswresample   3.  2.100 /  3.  2.100
  libpostproc    55.  2.100 / 55.  2.100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x2cb3600] decoding for stream 0 failed
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'video_only_1frame.mp4':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    creation_time   : 2015-10-03T12:28:39.000000Z
  Duration: 00:00:01.24, start: 0.000000, bitrate: 5762 kb/s
    Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080, SAR 1:1 DAR 16:9, 90k tbn, 180k tbc (default)
    Metadata:
      rotate          : 90
      creation_time   : 2015-10-03T12:28:39.000000Z
      handler_name    : VideoHandle
    Side data:
      displaymatrix: rotation of -90.00 degrees
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 152 kb/s (default)
    Metadata:
      creation_time   : 2015-10-03T12:28:39.000000Z
      handler_name    : SoundHandle
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> mpeg4 (native))
  Stream #0:1 -> #0:1 (aac (native) -> aac (native))
Press [q] to stop, [?] for help
Output #0, mov, to 'out.mov':
  Metadata:
    major_brand     : mp42
    minor_version   : 0
    compatible_brands: isommp42
    encoder         : Lavf58.17.101
    Stream #0:0(eng): Video: mpeg4 (mp4v / 0x7634706D), yuv420p, 1080x1920 [SAR 1:1 DAR 9:16], q=2-31, 200 kb/s, 1 fps, 16384 tbn, 1 tbc (default)
    Metadata:
      encoder         : Lavc58.23.100 mpeg4
      creation_time   : 2015-10-03T12:28:39.000000Z
      handler_name    : VideoHandle
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
      displaymatrix: rotation of -0.00 degrees
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 128 kb/s (default)
    Metadata:
      creation_time   : 2015-10-03T12:28:39.000000Z
      handler_name    : SoundHandle
      encoder         : Lavc58.23.100 aac
frame=    0 fps=0.0 q=0.0 Lsize=      13kB time=00:00:01.29 bitrate=  78.9kbits/s speed=43.2x
video:0kB audio:11kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 9.397406%
[aac @ 0x2cc0100] Qavg: 39785.051

The frame is dropped no matter the value for fps.