FIX: pointless error in log when failing to save post

rate_limit gets fired for rollback on in which case there is not post_number

app/models/post.rb

@@ -90,7 +90,7 @@ class Post < ActiveRecord::Base
   end
 
   def limit_posts_per_day
-    if user.first_day_user? && post_number > 1
+    if user.first_day_user? && post_number && post_number > 1
       RateLimiter.new(user, "first-day-replies-per-day", SiteSetting.max_replies_in_first_day, 1.day.to_i)
     end
   end