[MacPorts] #56334: ffmpeg @3.4.2: update to 4.0

[MacPorts]

#56334: ffmpeg @3.4.2: update to 4.0
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  Reporter:  l2dy    |      Owner:  dbevans
      Type:  update  |     Status:  assigned
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:  security
      Port:  ffmpeg  |
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Comment (by pmetzger):

 Zero King, even if you can't test it thoroughly, by doing a pull request,
 it will be possible for other people to check out your branch and test it.

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Ticket URL: <https://trac.macports.org/ticket/56334#comment:4>
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