Re-indexing of site stalled, & running cron won't advance
This problem was caused by discussion forum posts with a single comment from a user whose id was no longer in the user table (threads with more than one comment, even though one was from an invalid user id, did not cause the error). 

The fix was to find comments with invalid user ids, then change those ids in the Comment table to 0. 

This SQL query identified the problem user ids: 

SELECT DISTINCT a.uid FROM comment a left join users b on a.uid = b.uid 
WHERE b.uid IS NULL


With the problem user ids identified, you can either edit the table directly, or use UPDATE to change the column values. 

With those values changed in the comment table, the re-indexing proceeded to 100% without a hitch.