The thumbnail extractor should use a default image when the video is shorter than the seek offset

When you use the Automatic video thumbnailing option, and have a "Video seek offset for thumbnails" value which is greater than the length of the video, the thumbnail extractor fails to create a file.
Later in the video import process, Drupal displays the message Uploaded file is not a valid image. Only JPG, PNG and GIF files are allowed."

The thumbnail extractor should check the length of the video, and if smaller than the seek offset, should use a default picture.