pgsql: Add assertion about ssize_t narrowing in AIO code

Add assertion about ssize_t narrowing in AIO code

The result from pg_preadv() or pg_pwritev(), which is of type ssize_t,
is assigned to PgAioHandle.result, which is of type int. This should
be ok because the maximum result is limited by PG_IOV_MAX times
BLCKSZ. Add an assertion and a code comment to explain and check
this.

Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Discussion: https://www.postgresql.org/message-id/flat/f9aab072-0078-49e4-ab93-3b08086a4406(at)eisentraut(dot)org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/01f78a84bf5aca4c00fe1168381830aca6f1b873

Modified Files
--------------
src/backend/storage/aio/aio_io.c | 5 +++++
1 file changed, 5 insertions(+)