pgsql: Reset properly errno before calling write()

Reset properly errno before calling write()

6cb3372 enforces errno to ENOSPC when less bytes than what is expected
have been written when it is unset, though it forgot to properly reset
errno before doing a system call to write(), causing errno to
potentially come from a previous system call.

Reported-by: Tom Lane
Author: Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/31797.1533326676@sss.pgh.pa.us

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/afd5fde856675f54a7b5d857f7a173f9a3237b57

Modified Files
--------------
src/backend/access/heap/rewriteheap.c           | 1 +
src/backend/access/transam/twophase.c           | 1 +
src/backend/replication/logical/origin.c        | 3 +++
src/backend/replication/logical/reorderbuffer.c | 1 +
src/backend/replication/logical/snapbuild.c     | 1 +
src/backend/replication/slot.c                  | 1 +
src/bin/pg_basebackup/receivelog.c              | 2 ++
7 files changed, 10 insertions(+)