Blender Git Loki

Fix T43496: Infinite loop in kernel when using surface attribute for volume

The issue was caused bu the optimization in surface attributes for cases when
there's only a volume shader used. Some attributes doesn't make sense in that
case and were skipped from calculation.

However, it is possible that kernel would still try to access them (because of
the shader setup etc). Prevented an infinite loop in the kernel now, which
should not have much affect on regular renders.