Investigate and fix invalid question_multianswer sequence

Without the patch applied:

1. Create two course in the default category, "Course 1" and "Course 2"
2. Add a quiz to each course (don't add any questions)
3. Go to the question bank of Course 1 and create a cloze question in the "Default for Course 1" category with the following content:

   {1:SHORTANSWER:=Berlin} is the capital of Germany.

   {1:SHORTANSWER:=Canberra} is the capital of Australia.

4. Add the question to the quiz in Course 1
5. Go to the question bank for Course 1 and move the question to "Default for Category 1"
6. Add the question to the quiz in Course 2
7. Go to the question bank in Course 2 and move the question to "Default for Course 2"
8. Delete Course 2 - doing this will remove questions from the mdl_question table, but leave them referenced in sequence column of the mdl_question_multianswer table)
9. Create a backup of Course 1

With the patch applied (on a vanilla install):

1. Restore the backup from step 9 above
2. Verify there are no errors and the course is restored
3. Browse to the quiz in the restored course, preview it and verify a notification explaining that the question is corrupted displays, and that the text "This subquestion is missing from your system and cannot be displayed." is shown in place of the subquestion
4. Verify you are able to edit the quiz without any errors

With the patch applied (on a vanilla install):

1. Repeat the steps to make the backup from before
2. Go to Course 1 and verify the quiz is still working
3. Restore the new backup on a vanilla moodle without the patch applied, and verify it restores and the question in the quiz works (preview and edit)
4. Restore the new backup on a vanilla moodle with the patch applied and verify it restores and the question in the quiz works (preview and edit)