pg_checksums: Handle read and write returns correctly

pg_checksums: Handle read and write returns correctly

The read() return was not checking for errors, the write() return was
not checking for short writes.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/5de61b6b-8be9-7771-0048-860328efe027%402ndquadrant.com