MAPCAN/MAPCON Edge cases

Describe the bug

MAPCAN/MAPCON functions have two edge cases which are not correctly handled in cmucl.

To Reproduce & Expected behavior

(mapcan #'identity '(1 2 3))  ; should error, returns 3 instead
(mapcon #'car '(1 2 3))       ; same

(setf *print-circle* t)

;; This returns #1=(1 2 . #1#), which is correct
(let ((lst (list 1 2)))
  (flet ((f (x)
           (declare (ignore x))
           lst))
    (apply #'nconc (mapcar #'f '(1 2)))))

;; Should be equivalent to the previous example, but hangs instead.
(let ((lst (list 1 2)))
  (flet ((f (x)
           (declare (ignore x))
           lst))
    (mapcan #'f '(1 2))))

Also please see ansi-test/ansi-test#39 .

Desktop (please complete the following information):

• OS: Linux
• Version: CMU Common Lisp 21d (21D Unicode)