Fix unsafe usage of strerror(errno) within ereport().

This is the converse of the unsafe-usage-of-%m problem: the reason
ereport/elog provide that format code is mainly to dodge the hazard
of errno getting changed before control reaches functions within the
arguments of the macro.  I only found one instance of this hazard,
but it's been there since 9.4 :-(.

src/backend/libpq/auth.c

@@ -2041,10 +2041,12 @@ auth_peer(hbaPort *port)
 	pw = getpwuid(uid);
 	if (!pw)
 	{
+		int			save_errno = errno;
+
 		ereport(LOG,
 				(errmsg("could not look up local user ID %ld: %s",
 						(long) uid,
-						errno ? strerror(errno) : _("user does not exist"))));
+						save_errno ? strerror(save_errno) : _("user does not exist"))));
 		return STATUS_ERROR;
 	}