FIX: Return error if new topic category not found

If creating a topic via the api as an admin and the category you specify
cannot be found an error will now be returned instead of just creating
the topic with no category. This will prevent accidental public topic
creation originally intended for a private category.

lib/topic_creator.rb

@@ -122,6 +122,9 @@ class TopicCreator
 
     @guardian.ensure_can_create!(Topic, category) unless (@opts[:skip_validations] || @opts[:archetype] == Archetype.private_message)
 
+    if @opts[:category] && category.nil?
+      raise Discourse::NotFound
+    end
     topic_params[:category_id] = category.id if category.present?
 
     topic_params[:created_at] = Time.zone.parse(@opts[:created_at].to_s) if @opts[:created_at].present?