Fix attribute numbers in the response to BASE_BACKUP command.

In GPDB, there used to be an extra column in the result set. That was
removed as part of the walreplication work, so it now matches the
upstream. But forgot to update these PQgetvalue() calls in pg_basebackup
accordingly.

Based on @macroyuyang's original patch and diagnosis.

src/bin/pg_basebackup/pg_basebackup.c

@@ -870,7 +870,7 @@ ReceiveAndUnpackTarFile(PGconn *conn, PGresult *res, int rownum)
 	if (basetablespace)
 		strlcpy(current_path, basedir, sizeof(current_path));
 	else
-		strlcpy(current_path, PQgetvalue(res, rownum, 2), sizeof(current_path));
+		strlcpy(current_path, PQgetvalue(res, rownum, 1), sizeof(current_path));
 
 	/*
 	 * Get the COPY data
@@ -1441,15 +1441,15 @@ BaseBackup(void)
 	for (i = 0; i < PQntuples(res); i++)
 	{
 		if (showprogress)
-			totalsize += atol(PQgetvalue(res, i, 3));
+			totalsize += atol(PQgetvalue(res, i, 2));
 
 		/*
 		 * Verify tablespace directories are empty. Don't bother with the
 		 * first once since it can be relocated, and it will be checked before
 		 * we do anything anyway.
 		 */
-		if (format == 'p' && !PQgetisnull(res, i, 2))
-			verify_dir_is_empty_or_create(PQgetvalue(res, i, 2));
+		if (format == 'p' && !PQgetisnull(res, i, 1))
+			verify_dir_is_empty_or_create(PQgetvalue(res, i, 1));
 	}
 
 	/*