Guard reduced tree from errors

eth2/lmd_ghost/src/reduced_tree.rs

@@ -311,6 +311,7 @@ where
     /// become redundant and removed from the reduced tree.
     fn remove_latest_message(&mut self, validator_index: usize) -> Result<()> {
         if let Some(vote) = *self.latest_votes.get(validator_index) {
+            if self.nodes.contains_key(&vote.hash) {
                 self.get_mut_node(vote.hash)?.remove_voter(validator_index);
                 let node = self.get_node(vote.hash)?.clone();
 
@@ -357,6 +358,7 @@ where
 
                 self.latest_votes.insert(validator_index, Some(vote));
             }
+        }
 
         Ok(())
     }
@@ -370,7 +372,8 @@ where
     /// - it does not have any votes.
     fn maybe_delete_node(&mut self, hash: Hash256) -> Result<()> {
         let should_delete = {
-            let node = self.get_node(hash)?.clone();
+            if let Ok(node) = self.get_node(hash) {
+                let node = node.clone();
 
                 if let Some(parent_hash) = node.parent_hash {
                     if (node.children.len() == 1) && !node.has_votes() {
@@ -391,6 +394,10 @@ where
                     // A node without a parent is the genesis node and should not be deleted.
                     false
                 }
+            } else {
+                // No need to delete a node that does not exist.
+                false
+            }
         };
 
         if should_delete {