another crash fix

src/topics.js

@@ -272,8 +272,10 @@ var async = require('async'),
 							}
 							start = parseInt(start, 10);
 							for(var i=0; i<posts.length; ++i) {
+								if (posts[i]) {
 									posts[i].index = start + i;
 								}
+							}
 							if (posts[0]) {
 								posts[0].index = 0;
 							}