Don't write to stdin of a test process that could have already exited.

Instead, close that stdin.  Per buildfarm member conchuela.  Back-patch
to 9.6, where the test was introduced.

Discussion: https://postgr.es/m/26478.1555373328@sss.pgh.pa.us

src/test/recovery/t/017_shm.pl

@@ -151,7 +151,7 @@ like(slurp_file($gnat->logfile),
 my $single_stderr;
 ok( !run_log(
 		[ 'postgres', '--single', '-D', $gnat->data_dir, 'template1' ],
-		'<', \('SELECT 1 + 1'), '2>', \$single_stderr),
+		'<', \undef, '2>', \$single_stderr),
 	'live query blocks --single');
 print STDERR $single_stderr;
 like($single_stderr, $pre_existing_msg,