MacGui: check if the image returned by hb_get_preview2 is null to avoid a crash. The preview window will try to show the number of previews selected in the preferences, but an already loaded title might not have enough preview images.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6244 b64f7644-9d1e-0410-96f1-a4d463321fa5