bug#61629: master: Sometimes M-x imenu throws an error when there's a single function in the buffer.

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

On the master branch, the recipe below leads to an error being thrown
from imenu.

Awareness of this bug resulted from the thread Subject: Error in C++
Mode with Emacs 27.0.90, Date: Tue 24 Mar 2020, From: Angelo Graziosi.
This thread reported a bug in imenu, which never made it to debbugs, but
which was partially fixed.

Thanks to Dmitry Gutov, who made me aware of the bit that didn't get
fixed then.

To reproduce the error in the master branch:
(i) emacs -Q in a GUI environment.
(ii) Evaluate the following:

;; C/C++ modes
(defun my-c-mode ()
  "My customization for `c-mode' and `c++-mode'."
  ;; Add index of func. to menu bar
  (imenu-add-to-menubar "Functions"))

;; c++-mode
(add-hook 'c-mode-common-hook 'my-c-mode)

(setq imenu-auto-rescan t)
(setq imenu-use-popup-menu t)

(iii) Type the following into an empty C++ buffer called graziosi.cc:

int main ()
{
    return 0;
}

(iv) M-x imenu<RET>.

This throws the error "Wrong type argument: seqencep, #<marker at 1 in
graziosi.cc>".  This is a bug.

-- 
Alan Mackenzie (Nuremberg, Germany).