fix crash when you go to /topics/nonexistant

src/topics.js

@@ -20,6 +20,12 @@ var	RDB = require('./redis.js'),
 		var range_var = (category_id) ? 'categories:' + category_id + ':tid'  : 'topics:tid';
 
 		RDB.smembers(range_var, function(err, tids) {
+			
+			if(tids.length === 0) {
+				callback(false);
+				return;
+			}
+
 			var title = [],
 				uid = [],
 				timestamp = [],