fix a list infinite loop bug

imagine such code:
list_t ls1 = alloc_object(1);
cons(2, ls1);
these code will make an infinite loop, because NEXT(NEXT(ls1) ) == ls.
to avoid this bug, fix the alloc_object() function, add some statements.

C10-Elementary-Data-Structures/exercise_code/af-obj.c

@@ -31,6 +31,8 @@ list_t allocate_object() {
 
     list_t new = free_list;
     free_list = NEXT(free_list);
+    NEXT(new_obj) = empty_list;
+    PREV(new_obj) = empty_list;
     return new;
 }