merkletree.Append: fix appending to empty trees

merkletree/collapsed_tree.go

@@ -54,10 +54,12 @@ func (tree *CollapsedTree) Add(hash Hash) {
 }
 
 func (tree *CollapsedTree) Append(other *CollapsedTree) error {
+	if tree.size > 0 {
 		maxSize := uint64(1) << bits.TrailingZeros64(tree.size)
 		if other.size > maxSize {
 			return fmt.Errorf("tree of size %d is too large to append to a tree of size %d (maximum size is %d)", other.size, tree.size, maxSize)
 		}
+	}
 
 	tree.nodes = append(tree.nodes, other.nodes...)
 	tree.size += other.size