R1:8b9428b6dac2

Fix a possible undefined variable

Summary:
I pulled this out of the logs; not sure how anyone hit it, but this controller could bump into an undefined variable here:

```
2015/06/24 22:59:55 [error] 10150#0: *172493 FastCGI sent in stderr: "PHP message: [2015-06-24 22:59:55] EXCEPTION: (RuntimeException) Undefined variable: subscriber_phids at [<phutil>/src/error/PhutilErrorHandler.php:210]
PHP message: arcanist(head=master, ref.master=b697a3b80bdc), phabricator(head=redesign-2015, ref.master=0fd0f171f10f…