Since FileNotFound exceptions don't abort the provider search, opening
an invalid path would show errors such as "avisynth not found" when
Avisynth wasn't installed, even if Avisynth wasn't selected as the video
provider.
At least on OS X, allocating the buffers is one of the more expensive
parts of video playback, and on an arbitrary 720p H.264 file with simple
subtitles this cuts CPU usage while playing by about 30%.
Fixes errors when trying to automatically open audio from dummy video
and improves UX a little by disabling the Open Audio From Video menu
item when there are no audio tracks.