Cron task never finishes. Gets to line 'Looking for quiz overdue quiz attempts between Wednesday, 31 December 1969, 6:00 PM...' then never advances

Related to http://tracker.moodle.org/browse/MDL-34109

Cron script runs, but gets stuck at 'Looking for quiz overdue quiz attempts'. Checking SQL server, the cron is executing the following statements in an endless loop:

SELECT q.*, qc.contextid
FROM moodle.mdl_question q
JOIN moodle.mdl_question_categories qc ON q.category = qc.id
WHERE q.id = '4549'
(1 row returned)

SELECT * FROM moodle.mdl_question_hints WHERE questionid = '4549' ORDER BY id ASC
(0 rows returned)

SELECT * FROM moodle.mdl_question_multichoice WHERE question = '4549'.
(1 row returned)

SELECT * FROM moodle.mdl_question_answers WHERE question = '4549' ORDER BY id ASC
(4 rows returned)

Running https://github.com/moodle/moodle/blob/master/mod/quiz/cronlib.php#L91 by hand yields 140 records, so this isn't a huge database