Fix #35160: cycles was rendering glossy BSDF's with zero roughness too rough
after a bugfix for precision issues with low roughness. Now it renders them as
perfectly sharp which avoids the problematic calculations rather than increasing
the roughness.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56478 954f8c5b-7b00-dc11-b283-0030488c597c