Fix a bug where Phortune could fatal while building crumbs

Summary: Ref T12451. $this->getAccount() may not return an account.

Test Plan:

• Visit /phortune/X/, where X is the ID of an account you don't have permission to view.
• Before patch: fatal.
• After patch: normal policy exception page.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12451

Differential Revision: https://secure.phabricator.com/D17689