Add a guard against uninitialized default locale - Mailing list pgsql-hackers

Not known to be reachable after dbf217c1c7, but defend against similar
issues in the future. For instance, an extension might encounter the
problem by calling pg_newlocale_from_collation(DEFAULT_COLLATION_OID)
from _PG_init(), and end up with a NULL pointer dereference.

Backport through 17, though patch is different in 17 (also attached).

Regards,
    Jeff Davis