Fix delete rear and front pointer when there is only one element left

ex-3_21-23.scm

@@ -170,6 +170,9 @@
 (define (front-delete-deque! deque)
   (cond ((empty-deque? deque)
          (error "DELETE! called with an empty deque" deque))
+        ((null? (next-node (front-ptr deque)))
+         (set-front-ptr! deque '())
+         (set-rear-ptr! deque '()))
         (else
          (set-front-ptr! deque (next-node (front-ptr deque)))
          (set-prev-node! (front-ptr deque) '())
@@ -178,6 +181,9 @@
 (define (rear-delete-deque! deque)
   (cond ((empty-deque? deque)
          (error "DELETE! called with an empty deque" deque))
+        ((null? (prev-node (rear-ptr deque)))
+         (set-front-ptr! deque '())
+         (set-rear-ptr! deque '()))
         (else
          (set-rear-ptr! deque (prev-node (rear-ptr deque)))
          (set-next-node! (rear-ptr deque) '())
@@ -197,5 +203,7 @@
 (rear-delete-deque! d)
 (front-delete-deque! d)
 (rear-delete-deque! d)
+(front-delete-deque! d)
+(rear-delete-deque! d)
 (display-deque d)