It's not clear what the semantics of negative strides would be, so throw
an error instead.

Per report from Bauyrzhan Sakhariyev

Reviewed-by: Tom Lane, Michael Paquier
Discussion: https://www.postgresql.org/message-id/CAKpL73vZmLuFVuwF26FJ%2BNk11PVHhAnQRoREFcA03x7znRoFvA%40mail.gmail.com
Backpatch to v14