Fix a possible database ref fatal during MySQL setup checks if a host is unreachable

Summary:
Ref T12966. See that task for a description and reproduction steps.

If you put Phabricator in a master/replica configuration and then restart it, we may fatal here if the master is unreachable. Instead, we should survive setup checks.

Test Plan: Put Phabricator in a master/replica configuration, explicitly disabled the master by misconfiguring the port, restarted Phabricator. Before: fatal; after: login screen in read-only mode.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12966

Differential Revision: https://secure.phabricator.com/D18442