[FFmpegPostProcessor] Default of prefer ffmpeg

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`.

youtube_dl/postprocessor/ffmpeg.py

@@ -52,7 +52,7 @@ class FFmpegPostProcessor(PostProcessor):
 
     def _determine_executables(self):
         programs = ['avprobe', 'avconv', 'ffmpeg', 'ffprobe']
-        prefer_ffmpeg = self._downloader.params.get('prefer_ffmpeg', False)
+        prefer_ffmpeg = False
 
         self.basename = None
         self.probe_basename = None
@@ -60,6 +60,7 @@ class FFmpegPostProcessor(PostProcessor):
         self._paths = None
         self._versions = None
         if self._downloader:
+            prefer_ffmpeg = self._downloader.params.get('prefer_ffmpeg', False)
             location = self._downloader.params.get('ffmpeg_location')
             if location is not None:
                 if not os.path.exists(location):