Fix glitches introduced by nthcdr changes

* src/fns.c (Fnthcdr): Fix recently-introduced bug when
nthcdr is supposed to yield a non-nil non-cons.
Reported by Glenn Morris and by Pip Cet here:
https://lists.gnu.org/r/emacs-devel/2018-08/msg00699.html
https://lists.gnu.org/r/emacs-devel/2018-08/msg00708.html
Speed up nthcdr for small N, as suggested by Pip Cet here:
https://lists.gnu.org/r/emacs-devel/2018-08/msg00707.html
* test/src/fns-tests.el (test-nthcdr-simple): New test.

1 files changed, 5 insertions, 0 deletions

diff --git a/test/src/fns-tests.el b/test/src/fns-tests.el
index 92dc18fa034..b180f30f285 100644
--- a/test/src/fns-tests.el
+++ b/test/src/fns-tests.el
@@ -624,6 +624,11 @@
         (should (eq (gethash b2 hash)
                     (funcall test b1 b2)))))))
 
+(ert-deftest test-nthcdr-simple ()
+  (should (eq (nthcdr 0 'x) 'x))
+  (should (eq (nthcdr 1 '(x . y)) 'y))
+  (should (eq (nthcdr 2 '(x y . z)) 'z)))
+
 (ert-deftest test-nthcdr-circular ()
   (dolist (len '(1 2 5 37 120 997 1024))
     (let ((cycle (make-list len nil)))