FIX: never attempt to log invalid post numbers

Previously in some cases we would queue logging of invalid post numbers

The impact would be we would miss logging an incoming link and would leak
an error.

spec/requests/topics_controller_spec.rb

@@ -1095,6 +1095,18 @@ RSpec.describe TopicsController do
       end.to change(TopicViewItem, :count).by(1)
     end
 
+    it 'records a view to invalid post_number' do
+      user = Fabricate(:user)
+
+      expect do
+        get "/t/#{topic.slug}/#{topic.id}/#{256**4}", params: {
+          u: user.username
+        }
+        expect(response.status).to eq(200)
+      end.to change { IncomingLink.count }.by(1)
+
+    end
+
     it 'records incoming links' do
       user = Fabricate(:user)