pg_amcheck: Fix block number parsing on command line
The previous code wouldn't handle higher block numbers on systems
where sizeof(long) == 4.
Reviewed-by: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/
6a10a211-872b-3c4c-106b-
909ae5fefa61%40enterprisedb.com