Have llist_pop(0) return NULL the same way dlist_pop() does.

lib/llist.c

@@ -41,14 +41,13 @@ void llist_traverse(void *list, void (*using)(void *node))
 // as &list)
 void *llist_pop(void *list)
 {
-  // I'd use a void ** for the argument, and even accept the typecast in all
-  // callers as documentation you need the &, except the stupid compiler
-  // would then scream about type-punned pointers.  Screw it.
-  void **llist = (void **)list;
-  void **next = (void **)*llist;
+  void **llist = list, **next;
+
+  if (!list || !*llist) return 0;
+  next = (void **)*llist;
   *llist = *next;
 
-  return (void *)next;
+  return next;
 }
 
 // Remove first item from &list and return it