Fix sort procedure in base system

scheme-core/system.scm

@@ -1092,14 +1092,16 @@
                         (simple-sort grtr))))))
 
 (define (sort l (less? <) (key identity))
-  (if (or (null? l) (null? (cdr l))) l
+  (let sort ((l l))
+    (if (or (null? l) (null? (cdr l)))
+        l
         (let ((piv (car l)))
           (receive (less grtr)
               (separate (lambda (x) (less? (key x) (key piv)))
                         (cdr l))
             (nconc (sort less)
                    (list piv)
-                        (sort grtr))))))
+                   (sort grtr)))))))
 
 (define (apropos-list key)
   (let ((key (string-downcase