Test index-out-of-bounds in Features::to_context

When there are fewer known `from` feature bytes than known `to` feature
bytes, an index-out-of-bounds error can occur if the `from` features
have unknown features set in a byte past the greatest known `from`
feature byte.

lightning/src/ln/features.rs

@@ -301,7 +301,7 @@ mod sealed {
 		set_shutdown_any_segwit_required);
 
 	#[cfg(test)]
-	define_feature!(123456789, UnknownFeature, [NodeContext, ChannelContext],
+	define_feature!(123456789, UnknownFeature, [NodeContext, ChannelContext, InvoiceContext],
 		"Feature flags for an unknown feature used in testing.", set_unknown_feature_optional,
 		set_unknown_feature_required);
 }
@@ -774,6 +774,16 @@ mod tests {
 		assert!(!init_features.supports_gossip_queries());
 	}
 
+	#[test]
+	fn convert_to_context_with_unknown_flags() {
+		// Ensure the `from` context has fewer known feature bytes than the `to` context.
+		assert!(InvoiceFeatures::known().byte_count() < NodeFeatures::known().byte_count());
+		let invoice_features = InvoiceFeatures::known().set_unknown_feature_optional();
+		assert!(invoice_features.supports_unknown_bits());
+		let node_features: NodeFeatures = invoice_features.to_context();
+		assert!(!node_features.supports_unknown_bits());
+	}
+
 	#[test]
 	fn set_feature_bits() {
 		let features = InvoiceFeatures::empty()