BUG #15449: file_fdw using program cause exit code error when usingLIMIT

The following bug has been logged on the website:

Bug reference:      15449
Logged by:          Eric Cyr
Email address:      eric.cyr@gmail.com
PostgreSQL version: 10.5
Operating system:   Ubuntu 16.04.2 LTS (Xenial Xerus)
Description:

CREATE FOREIGN TABLE test_file_fdw_program_limit
    (
      message text
    )
    SERVER pg_log_fdw OPTIONS (program 'echo "test"', format 'csv')
;

--

SELECT * FROM test_file_fdw_program_limit;
/*
OK
*/

SELECT * FROM test_file_fdw_program_limit LIMIT 2;
/*
OK
*/

SELECT * FROM test_file_fdw_program_limit LIMIT 0;
/*
[38000] ERROR: program "echo "test"" failed Detail: child process exited
with exit code 1
*/

--

LIMIT 0 is the easiest way we found to reproduce the error, but we
encountered same issue with LIMIT (< file lines) on a program doing an if
exists + file output stream manipulation.