list: fix unused var warning.

/home/rusty/devel/cvs/ccan/ccan/list/list.h: In function ‘main’:
/home/rusty/devel/cvs/ccan/ccan/list/list.h:280:15: warning: variable ‘first’ set but not used [-Wunused-but-set-variable]

/home/rusty/devel/cvs/ccan/ccan/list/list.h: In function ‘main’:
/home/rusty/devel/cvs/ccan/ccan/list/list.h:304:15: warning: variable ‘last’ set but not used [-Wunused-but-set-variable]
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/list/list.h

@@ -279,6 +279,8 @@ static inline void list_del_from(struct list_head *h, struct list_node *n)
  * Example:
  *	struct child *first;
  *	first = list_top(&parent->children, struct child, list);
+ *	if (!first)
+ *		printf("Empty list!\n");
  */
 #define list_top(h, type, member)					\
 	((type *)list_top_((h), list_off_(type, member)))
@@ -301,6 +303,8 @@ static inline const void *list_top_(const struct list_head *h, size_t off)
  * Example:
  *	struct child *last;
  *	last = list_tail(&parent->children, struct child, list);
+ *	if (!last)
+ *		printf("Empty list!\n");
  */
 #define list_tail(h, type, member) \
 	((type *)list_tail_((h), list_off_(type, member)))