shardtree: Derive correct shard root addrs in ShardTree::insert_tree

`LocatedTree::decompose_to_level` will return the tree as-is if it is
smaller than a shard, so we can't assume that the address of `subtree`
is a valid shard address.

shardtree/src/lib.rs

@@ -587,7 +587,11 @@ impl<
     ) -> Result<Vec<IncompleteAt>, ShardTreeError<S::Error>> {
         let mut all_incomplete = vec![];
         for subtree in tree.decompose_to_level(Self::subtree_level()).into_iter() {
-            let root_addr = subtree.root_addr;
+            // `LocatedTree::decompose_to_level` will return the tree as-is if it is
+            // smaller than a shard, so we can't assume that the address of `subtree` is a
+            // valid shard address.
+            let root_addr = Self::subtree_addr(subtree.root_addr.position_range_start());
+
             let contains_marked = subtree.root.contains_marked();
             let (new_subtree, mut incomplete) = self
                 .store