Make uselocale protection more consistent

This is a patch which implements an issue discussed in bug #17946[0]. It
doesn't fix the overarching issue of the bug, but merely a consistency
issue which was found while analyzing code by Heikki. I had originally
submitted the patch within that thread, but for visibility and the
purposes of the commitfest, I have re-sent it in its own thread.

[0]: https://www.postgresql.org/message-id/49dfcad8-90fa-8577-008f-d142e61af46b@iki.fi

--
Tristan Partin
Neon (https://neon.tech)