verify before streaming from map channel

app/channels/map_channel.rb

@@ -2,7 +2,7 @@ class MapChannel < ApplicationCable::Channel
   # Called when the consumer has successfully
   # become a subscriber of this channel.
   def subscribed
-    # TODO: verify permission
+    return unless Pundit.policy(current_user, Map.find(params[:id])).show?
     stream_from "map_#{params[:id]}"
   end
 end