Selecting the about to be deleted frame in a delete-frame-function
aborts Emacs, because this change goes undetected by the function
delete_frame.  The following lisp code should produce this behaviour.

(add-hook 'delete-frame-functions 'select-frame)
(let ((f1 (make-frame))
      (f2 (make-frame)))
  (select-frame f2)
  (delete-frame f1)
  (delete-frame f2))

At the very top, delete_frame reads the selected frame into the variable
sf, then...

   struct frame *f = decode_any_frame (frame);
   struct frame *sf = SELECTED_FRAME ();
   ...
   safe_call2 (Qrun_hook_with_args, Qdelete_frame_functions, frame)
   // does effectively SELECTED_FRAME = f and thus
   ...
   /* Don't let the frame remain selected.  */
   if (f == sf) // is false, but sf does not actually hold the
                // SELECTED_FRAME anymore.

So, the dead frame f remains selected and this leads to an
emacs_abort, the next time SELECTED_FRAME is invoked.

Solution: Read SELECTED_FRAME into sf before it's first use, but after
calling the hook.