For a quiz with more than 10 questions, remove page break then add section heading fails

Thanks to Antonia Bonaccorso for reporting this in the forum, and patiently explaining to me how to reproduce it.

1. On https://qa.moodle.net/course/view.php?id=7, or any new course, as teacher.
2. Add 11 questions to a quiz, one on each page.
3. Use the remove page-break icon to remove the page-break between pages 10 and 11.
4. From the Add menu for page 10, choose a new section heading.

Expected result: page reloads with the new head.

Actual result: Error: Column 'firstslot' cannot be null

If you look at the HTTP traffic, you can see the problem is a request /mod/quiz/edit.php?cmid=977&addsectionatpage=100 (when it should be 10).

The problem is, the JavaScript that updates the page after altering page-breaks incorrectly handles two-digit page-breaks.