Blender Git Loki

Cycles X: Fix possible use of uninitialized ShaderClosure

It is possible that BSDF allocation will advance pointer in the
allocation "pool" but will return null pointer if the weight is
too small.

One artist-measurable issue this change fixes is random issues
with denoising: normal pass for denoising could have accessed
non-initialized normal of a closure.

Differential Revision: https://developer.blender.org/D11230