Fix an errant "switch ... continue"

Summary:
See https://discourse.phabricator-community.org/t/unhandled-exception-on-create-task/2062.

This construction has the same behavior as "switch ... break" but is unconventional. PHP 7.3 started warning about it because it's likely a mistake.

Test Plan: Created a task, edited a task owner. The new code is functionally identical to the old code.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19772