[O] Bug: imenu fails with non-continuous structure [9.1.13 (release_9.1.13 @ /home/thblt/.emacs.d/lib/org/lisp/)]

[Thibault Polge]

Steps to reproduce, from emacs -q:

 1. (setq org-imenu-depth 10)

 2. Create the following document:

#+begin_src org
#+TITLE: Test

* Level 1
** Level 2
*** Level 3
**** Level 4
******** Level 8
#+end_src

Notice the break on the structure: levels 5, 6 and 7 are missing.

 3. M-x imenu, select each Level n entry

 4. imenu will crash after you've selected Level 4

I can reproduce this from emacs -q using org-mode from git master
(842002f9f7e3246cc285cc9bca1adb715120438d).  I'm running 26.1 on NixOS.

(I didn't include the detailed system report since I've tested on
multiple setups, including -q.  Please ask if you need any extra
information)

Thanks
Thibault