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
```common_lisp
(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 https://gitlab.common-lisp.net/ansi-test/ansi-test/-/issues/39 .
## Desktop (please complete the following information):
- OS: Linux
- Version: CMU Common Lisp 21d (21D Unicode)
issue