Fixes for context updates of the node editor:
* If the node tree can be updated from context (tree has get_from_context callback defined), reset the pointers first to clear the editor path if no tree can be found.
* Stupid mistake: snode->from != snode->from is always false.
* Shader nodes context update: set the 'from' pointer to the active object, even if it doesn't have a material or node tree.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55408 954f8c5b-7b00-dc11-b283-0030488c597c