FIX: don't error topic RSS when posts are deleted (#11641)

app/controllers/topics_controller.rb

@@ -840,6 +840,8 @@ class TopicsController < ApplicationController
   end
 
   def feed
+    raise Discourse::NotFound if !Post.exists?(topic_id: params[:topic_id])
+
     @topic_view = TopicView.new(params[:topic_id])
     discourse_expires_in 1.minute
     render 'topics/show', formats: [:rss]