simplehash: preserve consistency in case of OOM.

simplehash: preserve consistency in case of OOM.

Compute size first, then allocate, then update the structure.

Previously, an out-of-memory when growing could leave the hashtable in
an inconsistent state.

Discussion: https://postgr.es/m/20231117201334.eyb542qr5yk4gilq@awork3.anarazel.de
Reviewed-by: Andres Freund
Reviewed-by: Gurjeet Singh