Blender Git Loki

Fix possible NaN throughput in Cycles random walk

Based on asymptotic values of formulas when diffuse length is reaching
1 the throughput will be going to 0. So terminate the path early on
without causing numerical issues.

Just something I've notices with the Junk Shop scene while running
debugger for something else.

Differential Revision: https://developer.blender.org/D11237