reports: fix exception when deleting reported comments or forum posts.

Fix regression in 1a4efbda3. Locking the comment before validation
failed when the comment had unsaved changes, as is the case when
clearing reports from a comment before it is deleted.

app/models/moderation_report.rb

@@ -8,7 +8,7 @@ class ModerationReport < ApplicationRecord
   belongs_to :model, polymorphic: true
   belongs_to :creator, class_name: "User"
 
-  before_validation { model.lock! }
+  before_validation(on: :create) { model.lock! }
   validates :reason, presence: true
   validates :model_type, inclusion: { in: MODEL_TYPES }
   validates :creator, uniqueness: { scope: [:model_type, :model_id], message: "have already reported this message." }, on: :create