Fix #37175, Viewer node issue for newly toggled render passes.
The RenderLayers node would use the "combined" image result for all passes which don't have a valid render result yet. This causes problems when the buffer element size is not actually 4 floats (RGBA) as
with the 3 float normal passes. Also the result is rather meaningless then, so just keep the image buffer at NULL for unavailable passes, which will return plain (0,0,0) color.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60907 954f8c5b-7b00-dc11-b283-0030488c597c