Fix #37192, Rendered preview causes crash when deleting a material node in shader node editor. The 'free' callback for node execution data was accessed from the node->typeinfo, but this pointer can
become invalid because the render database is not immediately freed after the job finishes. To avoid access to dangling node pointers, store the function callback in the exec data itself. The node
pointer must not be accessed in the free function (wasn't used before either), these functions are purely for the execution data.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60997 954f8c5b-7b00-dc11-b283-0030488c597c