WinGui: Don't grab previews when the preview window hasn't been opened.  (Note, there is still a crash that sometimes happens for some resolutions that needs fixed, but the UI shouldn't crash anymore if the window hasn't been opened)

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6205 b64f7644-9d1e-0410-96f1-a4d463321fa5