Fix method call on possible null

Summary: Fixes T10317. If we failed to load $device, it will be null, so we won't be able to call getName() on it.

Test Plan: SSH_CLIENT='127.0.0.1' ./bin/ssh-exec --phabricator-ssh-device xyz no longer fatals

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10317

Differential Revision: https://secure.phabricator.com/D15235