Add a guard against uninitialized default locale

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