posts: add workaround to avoid falsely delete pending posts.

Add a temporary workaround for the database index corruption bug. Add a
check to skip deleting pending posts if they're not really pending.

app/logical/post_pruner.rb

@@ -10,12 +10,14 @@ module PostPruner
 
   def prune_pending!
     Post.pending.expired.each do |post|
+      next unless post.is_pending?
       post.delete!("Unapproved in three days", user: User.system)
     end
   end
 
   def prune_flagged!
     PostFlag.expired.each do |flag|
+      next unless flag.post.is_flagged?
       flag.post.delete!("Unapproved in three days after returning to moderation queue", user: User.system)
     end
   end