Blender Git Loki

Cycles: Don't perform re-intersection if ray distance is zero

It is possible that ray distance will be zero which would make intersection
refinement return NaN as the refined position which would later lead to all
sort of mathematical issues.

Don't think there are ways to improve intersection accuracy for such rays
so just return original intersection coordinate.

This should fix T43475.

TODO: Need to look into possible issues in Ashikhmin BSDF which might return
zero-length reflected/transmitted ray?