fix(beatree): prevent underflow on deletion of non-existend key

nomt/src/beatree/ops/update/leaf_stage.rs

@@ -231,7 +231,7 @@ fn filter_leaves_changeset(leaf_changeset: &mut Vec<(Key, Option<PageNumber>)>)
     // The deleted page was already inserted in the freed_pages, thus,
     // now only the inserted page must be kept, resulting in a standard
     // modification of the same page, but made by two workers.
-    for i in 0..leaf_changeset.len() - 1 {
+    for i in 0..leaf_changeset.len().saturating_sub(1) {
         if leaf_changeset[i].0 == leaf_changeset[i + 1].0 {
             // PANICS: If two changesets refer to the same page, they
             // are expected to be treated as a single one, with one

nomt/tests/add_remove.rs

@@ -54,3 +54,19 @@ fn add_remove_1000() {
         assert_eq!(root, expected_roots[10 - i - 1]);
     }
 }
+
+#[test]
+fn delete_missing_key_in_empty_db() {
+    let mut t = Test::new("delete_missing_key_in_empty_db");
+    t.write([0; 32], None);
+    assert_eq!(t.commit().0.into_inner(), Node::default());
+}
+
+#[test]
+fn delete_missing_key_in_non_empty_db() {
+    let mut t = Test::new("delete_missing_key_in_non_empty_db");
+    common::set_balance(&mut t, 0, 1000);
+    let root = t.commit().0;
+    common::kill(&mut t, 1);
+    assert_eq!(t.commit().0, root);
+}