MacGui: Fix a null placeholder upon launch in the x264 unparse string.
- Patch by sanmarcos Thanks!
- Explained here https://reviews.handbrake.fr/r/426/

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5213 b64f7644-9d1e-0410-96f1-a4d463321fa5