Added missing node->id pointer checks in case node groups have unavailable node_tree data blocks.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55513 954f8c5b-7b00-dc11-b283-0030488c597c
code cleanup: shadowing, adjust type.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55514 954f8c5b-7b00-dc11-b283-0030488c597c