Otherwise crashes when trying to generate thumbnail for a video file without actual video data.

Applications/18.08 branch

I tried to fix all places where it would blow up when doing so (MovieDecoder::seek, then MovieDecoder::calculateDimensions, then somewhere else) but easiest would be to just bail out when it failed to even generate a frame in the first place.

• No longer have thumbnailer crash when I have a .mpeg file that is actually an audio file and not a video file
• My other video thumbnails still generate correctly