FIX: avoid validations when destroying posts (#16049)

Previously email validations could fire when deleting posts if for
certain reasons any user validations fail on the user objects

This kind of condition could happen in core due to a corruption of a
user record, or via a plugin that introduces a new validation on User

lib/post_destroyer.rb

@@ -388,8 +388,7 @@ class PostDestroyer
     UserStatCountUpdater.decrement!(@post)
 
     if @post.created_at == author.last_posted_at
-      author.last_posted_at = author.posts.order('created_at DESC').first.try(:created_at)
-      author.save!
+      author.update_column(:last_posted_at, author.posts.order('created_at DESC').first.try(:created_at))
     end
 
     if @post.is_first_post? && @post.topic && !@post.topic.private_message?

spec/lib/post_destroyer_spec.rb

@@ -185,6 +185,18 @@ describe PostDestroyer do
         .and change { post.topic.user_id }.to(Discourse.system_user.id)
     end
 
+    it "bypassed validation when updating users" do
+      post = create_post
+
+      # ensure user would fail validations
+      UserEmail.where(user_id: post.user_id).delete_all
+
+      PostDestroyer.new(admin, post.reload).destroy
+      PostDestroyer.new(admin, post.reload, force_destroy: true).destroy
+
+      expect(Post.with_deleted.find_by(id: post.id)).to eq(nil)
+    end
+
     describe "post_count recovery" do
       before do
         post