Fix for #35591:
* Cycles Mix closure could render strange effects, when the user entered a value out of the 0...1 range. This was already clamped for OSL, clamp for SVM as well. 

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57178 954f8c5b-7b00-dc11-b283-0030488c597c