Fix issues in binary_upgrade_logical_slot_has_caught_up().
The commit
29d0a77fa6 labelled binary_upgrade_logical_slot_has_caught_up()
as a non-strict function to allow providing a better error message to callers
in case the passed slot_name is NULL. On further discussion, it seems that
it is not helpful to have a different error message for NULL input in this
function, so this patch marks the function as strict.
This patch also removes the explicit permission check to use replication
slots as this function is invoked only by superusers and instead adds an
Assert.
Reported-by: Masahiko Sawada
Author: Hayato Kuroda
Reviewed-by: Vignesh C
Discussion: https://postgr.es/m/CAD21AoDSyiBKkMXBxN_gUayZZUCOgyHnG8Ge8rcPXNP3Tf6B4g@mail.gmail.com