tlist: remove unused var warning in example.

Compiler:
/home/rusty/devel/cvs/ccan/ccan/tlist/tlist.h: In function ‘main’:
/home/rusty/devel/cvs/ccan/ccan/tlist/tlist.h:186:15: warning: variable ‘first’ set but not used [-Wunused-but-set-variable]

Compiler:
/home/rusty/devel/cvs/ccan/ccan/tlist/tlist.h: In function ‘main’:
/home/rusty/devel/cvs/ccan/ccan/tlist/tlist.h:205:15: warning: variable ‘last’ set but not used [-Wunused-but-set-variable]
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/tlist/tlist.h

@@ -185,6 +185,8 @@
  * Example:
  *	struct child *first;
  *	first = tlist_top(&parent->children, list);
+ *	if (!first)
+ *		printf("Empty list!\n");
  */
 #define tlist_top(h, member)						\
 	((tcon_type((h), canary))					\
@@ -202,6 +204,8 @@
  * Example:
  *	struct child *last;
  *	last = tlist_tail(&parent->children, list);
+ *	if (!last)
+ *		printf("Empty list!\n");
  */
 #define tlist_tail(h, member)						\
 	((tcon_type((h), canary))					\