Fix probable copy-paste error in list iterator, from end

The following code failed:

     (sb-sequence:with-sequence-iterator-functions
         (next stop value _set _index _copy)
         ('(a b c d) :from-end t)
       (loop until (stop) collect (value) do (next)))

... with the following error:

    The value
      (D)
    is not of type
      NUMBER
    when binding SB-KERNEL::X
       [Condition of type TYPE-ERROR]

src/pcl/sequence.lisp

@@ -216,8 +216,7 @@
                   (if (eq iterator list)
                       *exhausted*
                       (do* ((cdr list (cdr cdr)))
-                           ((eq (cdr cdr) iterator) cdr)))
-                  (1+ iterator))
+                           ((eq (cdr cdr) iterator) cdr))))
                 (lambda (list iterator from-end)
                   (declare (ignore list from-end))
                   (cdr iterator)))

tests/extended-sequences.impure.lisp

@@ -85,3 +85,13 @@
 (with-test (:name (concatenate :result-type class))
   (assert (typep (concatenate (find-class 'extended-sequence) '(1 2) '(3 4))
                  'extended-sequence)))
+
+(with-test (:name :list-iterator-from-end)
+  (checked-compile-and-assert
+   ()
+   `(lambda (x)
+      (sb-sequence:with-sequence-iterator-functions
+          (next stop value _set _index _copy)
+          (x :from-end t)
+        (loop until (stop) collect (value) do (next))))
+   (('(a b c d)) '(d c b a) :test #'equal)))