dv muxer accepts all framerates

[Carl Eugen Hoyos]

As reported by forum user tygerbug, libavformat does not warn if an incorrect framerate is used when writing a dv file:

$ ffmpeg -f lavfi -i testsrc=s=720x480:rate=10 -pix_fmt yuv411p -y -t 20 out.dv
ffmpeg version N-83100-ge109c54 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      55. 43.100 / 55. 43.100
  libavcodec     57. 72.100 / 57. 72.100
  libavformat    57. 62.100 / 57. 62.100
  libavdevice    57.  2.100 / 57.  2.100
  libavfilter     6. 69.100 /  6. 69.100
  libswscale      4.  3.101 /  4.  3.101
  libswresample   2.  4.100 /  2.  4.100
  libpostproc    54.  2.100 / 54.  2.100
Input #0, lavfi, from 'testsrc=s=720x480:rate=10':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: rawvideo (RGB[24] / 0x18424752), rgb24, 720x480 [SAR 1:1 DAR 3:2], 10 tbr, 10 tbn, 10 tbc
Output #0, dv, to 'out.dv':
  Metadata:
    encoder         : Lavf57.62.100
    Stream #0:0: Video: dvvideo, yuv411p, 720x480 [SAR 1:1 DAR 3:2], q=2-31, 200 kb/s, 10 fps, 10 tbn, 10 tbc
    Metadata:
      encoder         : Lavc57.72.100 dvvideo
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> dvvideo (native))
Press [q] to stop, [?] for help
frame=  200 fps=0.0 q=-0.0 Lsize=   23438kB time=00:00:20.00 bitrate=9600.0kbits/s speed=53.6x
video:23438kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.000000%