fix for valgrind warning - using uninitialized variable. Though in practice it didn't cause any problems.

Getting the 'CTX_data_main' while un-fullscreen'ing a space would do a context lookup into the view3d_context which had an uninitialized layer.
since view3d_context doesn't hold a 'main' member it never did anything but cleaner not to do context lookups while modifying the view.
- noticed while checking on a real bug :)


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54311 954f8c5b-7b00-dc11-b283-0030488c597c