issue with proc being None

[gyglim]

I have an issue coming from #630:

Stack trace:

  File "/usr/local/lib/python2.7/dist-packages/moviepy/Clip.py", line 514, in __exit__
    self.close()
  File "/usr/local/lib/python2.7/dist-packages/moviepy/video/io/VideoFileClip.py", line 125, in close
    self.reader.close()
  File "/usr/local/lib/python2.7/dist-packages/moviepy/video/io/ffmpeg_reader.py", line 182, in close
    self.proc.terminate()
AttributeError: 'NoneType' object has no attribute 'terminate'

Comes from https://github.com/Zulko/moviepy/blame/master/moviepy/video/io/ffmpeg_reader.py#L186
before the PR there was del self.proc instead. We should go back to that or initialize self.proc = None

Sample to reproduce:

with VideoFileClip(YOUR_VIDEO) as v:    
    with v.subclip(0,1) as s:
       pass