Fix for a node compatibility code bug (reported by kopias in IRC). Compatibility data (links with NULL fromnode/tonode pointers) was created for all node trees in library data, including local scene/material/... trees, but on loading was only actually removed for node groups. Now cleans up all trees in the library to be sure, and only generate new compat data for actual group trees.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55478 954f8c5b-7b00-dc11-b283-0030488c597c