Fetching null "data" for tid 0 may cause duplicate, malformed lineage

Issue Summary

In {taxonomy_hierarchy}, top-level terms have entries with the parent ID set to 0. The recursive term update function in this module, lineage_update_term_r(), attempts to fetch data for this "fake" parent term, which results a lineage being added for term 0; subsequently duplicate, malformed lineages like the following are created for each term:

The attached patch resolves this issue by discarding these "empty" data rows for terms that don't exist. (Note: I've made the patch against 6.x-1.0 release instead of the current dev build or HEAD because, somehow, the release is nine months newer than either?)