use more cautious array index access

Author: arik-so

lightning-rapid-gossip-sync/src/processing.rs

@@ -191,7 +191,10 @@ impl<NG: Deref<Target=NetworkGraph<L>>, L: Deref> RapidGossipSync<NG, L> where L
 				if feature_detail_marker > 0 {
 					if feature_detail_marker < 7 {
 						let feature_index = (feature_detail_marker - 1) as usize;
-						synthetic_node_announcement.features = default_node_features[feature_index].clone();
+						synthetic_node_announcement.features = default_node_features
+							.get(feature_index)
+							.ok_or(DecodeError::InvalidValue)?
+							.clone();
 					} else {
 						let node_features: NodeFeatures = Readable::read(read_cursor)?;
 						synthetic_node_announcement.features = node_features;