response code must be 200 (#2916)

app/controllers/concerns/joiner.rb

@@ -53,6 +53,7 @@ module Joiner
     http = Net::HTTP.new(uri.host, uri.port)
     http.use_ssl = true if uri.scheme == 'https'
     response = http.request_head(uri)
+    return false if response.code != "200"
     return response['content-length'].to_i < Rails.configuration.max_avatar_size
   end