Fix for NULL pointer bug, reported by Jens Verwiebe in IRC. ED_node_tag_update_id tries to get a node tree from the active id pointer, but this only works for standard node types, not for pynodes.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56193 954f8c5b-7b00-dc11-b283-0030488c597c