Fix for Render Layers node, reported by Jason van Gumster (Fweeb) on IRC. The render layer operations need their RenderData pointer set. This was done for all connected output operations, but not for the preview operation which uses the image color layer. In case the image output is unconnected this leads to NULL pointer access.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55481 954f8c5b-7b00-dc11-b283-0030488c597c