Fix fuzz failure

Author: arik-so

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

@@ -54,7 +54,7 @@ pub(crate) fn update_network_graph_from_byte_stream<R: Read>(
 	let chain_hash: BlockHash = Readable::read(read_cursor)?;
 	let latest_seen_timestamp: u32 = Readable::read(read_cursor)?;
 	// backdate the applied timestamp by a week
-	let backdated_timestamp = latest_seen_timestamp - 24 * 3600 * 7;
+	let backdated_timestamp = latest_seen_timestamp.saturating_sub(24 * 3600 * 7);
 
 	let node_id_count: u32 = Readable::read(read_cursor)?;
 	let mut node_ids: Vec<PublicKey> = Vec::with_capacity(std::cmp::min(