pgsql: Fixed signed/unsigned mismatch in test_dsm_registry.

Fixed signed/unsigned mismatch in test_dsm_registry. Oversight in commit 8b2bcf3f28. Reviewed-by: Masahiko Sawada Discussion: https://postgr.es/m/aECi_gSD9JnVWQ8T%40nathan Backpatch-through: 17 Branch -- REL_17_STABLE Details --- https://git.postgresql.org/pg/commitdiff/65145d0955c494d

pgsql: Fixed signed/unsigned mismatch in test_dsm_registry.

Fixed signed/unsigned mismatch in test_dsm_registry. Oversight in commit 8b2bcf3f28. Reviewed-by: Masahiko Sawada Discussion: https://postgr.es/m/aECi_gSD9JnVWQ8T%40nathan Backpatch-through: 17 Branch -- master Details --- https://git.postgresql.org/pg/commitdiff/304862973e9a77c31cbf05