Discovered by meowmiomew while testing MDLQA-18012.

After restoring the backup file and starting an attempt on "A listening quiz" the following error is displayed:

From timhunt on the QA chat: It is blowing up here

But, really, that is because a few lines before, there is a call to $quizobj->preload_questions();, which does mod_quiz\question\bank\qbank_helper::get_question_structure($this->quiz->id, $this->context); which is basically one big DB query.

And, that will be returning the wrong thing, presumably because the backup failed to covert the 1.9 data correctly.