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

scripts/ssh/ssh-exec.php

@@ -129,7 +129,7 @@ try {
       throw new Exception(
         pht(
           'Invalid device name ("%s"). There is no device with this name.',
-          $device->getName()));
+          $device_name));
     }
 
     // We're authenticated as a device, but we're going to read the user out of