message: make_tree: Add explicit panic on empty list
diff: @@ -39,7 +39,9 @@ impl MerkleNode {
  fn make_tree<T>(xs: &[T]) -> Self
  where T: se::Serialize
  {
  - if xs.len() == 1 {
  + if xs.is_empty() {
  + panic!("make_tree applied to empty list")
  + } else if xs.len() == 1 {
  let mut bs = vec![0u8];
  xs[0].serialize(se::Serializer::new(&mut bs)).unwrap();
  MerkleNode::Leaf(Hash::new(&bs))
  