Hello, I trust that this is the right place to report a segfault.

Using whatever is current in debian 9, emacs segfaults
from a minimum setup when using an empty string for org-ellipsis.

Here's how to reproduce it:

> emacs -Q

(setq org-ellipsis "") C-x C-e
(org-mode)  C-x C-e
* asd
** def C-p tab  < and boom

My emacs isn't built from sources, so the ftrace/backtrace
is meaningless for the "Fatal error 11: Segmentation fault"

If there is a bugtracker and there always is a bugtracker,
then my apologies in case this has already been dealt with.