When no `downloader` is passed to `FFmpegPostProcessor`
an exception was raised trying to get the prefer ffmpeg param.
    AttributeError: 'NoneType' object has no attribute 'params'
This fixes and defaults to `False`.
		
	
When no `downloader` is passed to `FFmpegPostProcessor`
an exception was raised trying to get the prefer ffmpeg param.
    AttributeError: 'NoneType' object has no attribute 'params'
This fixes and defaults to `False`.