Fixed ~duck crashing Clive because it's not an actual duck game command.

src/games/duck.js

@@ -56,6 +56,11 @@ function onCommand(args, context) {
 		return;
 	}
 
+	if (!duck) {
+		// unrecognized command
+		return;
+	}
+
 	const hit = Math.random() > config.duck.missRatio;
 	const time = new Date().getTime() - duck.getTime();