Fixup for prefetching support on macOS

The new code path (commit 6654bb9204) should call FileAccess() first,
like the posix_fadvise() path.

Reported-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/0827edec-1317-4917-a186-035eb1e3241d%40eisentraut.org

src/backend/storage/file/fd.c

@@ -2110,6 +2110,10 @@ retry:
 		}			ra;
 		int			returnCode;
 
+		returnCode = FileAccess(file);
+		if (returnCode < 0)
+			return returnCode;
+
 		ra.ra_offset = offset;
 		ra.ra_count = amount;
 		pgstat_report_wait_start(wait_event_info);