Don't crash while scanning when we can't get previews for some title.

Don't crash while scanning when we can't get previews for some title.
 - if we get no previews for some title, ignore the title
 - use the most common aspect ratio found from our 10 previews rather than just the aspect of preview 2 (otherwise we'll crash in hb_fix_aspect if we don't get preview 2).
 - check parameters in hb_fix_aspect so we don't divide by zero.

git-svn-id: svn://localhost/HandBrake/trunk@1322 b64f7644-9d1e-0410-96f1-a4d463321fa5