modreports: fix private user information leak in new modreport action.

Fix this:

    https://danbooru.donmai.us/moderation_reports/new.json?moderation_report[model_id]=52664&moderation_report[model_type]=User

raising an `undefined method `reportable?' for #<UserPolicy ...>`
exception, which contained the full user object in the error message,
which leaked private user information.

app/policies/moderation_report_policy.rb

@@ -10,7 +10,7 @@ class ModerationReportPolicy < ApplicationPolicy
   end
 
   def create?
-    unbanned? && policy(record.model).reportable?
+    unbanned? && policy(record.model).try(:reportable?)
   end
 
   def update?