child_process: Check stderr before accessing it

If something bad happens in spawnSync, stderr might be null. Therefore,
we have to check it before using it, so we won't mask the actual
exception.

Ref: #9152
PR-URL: #6877
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Robert Jefe Lindstädt <robert.lindstaedt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/child_process.js

@@ -479,7 +479,7 @@ function execFileSync(/*command, args, options*/) {
 
   var ret = spawnSync(opts.file, opts.args.slice(1), opts.options);
 
-  if (inheritStderr)
+  if (inheritStderr && ret.stderr)
     process.stderr.write(ret.stderr);
 
   var err = checkExecSyncError(ret);
@@ -499,7 +499,7 @@ function execSync(/*command, options*/) {
   var ret = spawnSync(opts.file, opts.args, opts.options);
   ret.cmd = opts.cmd;
 
-  if (inheritStderr)
+  if (inheritStderr && ret.stderr)
     process.stderr.write(ret.stderr);
 
   var err = checkExecSyncError(ret);

test/sequential/test-child-process-execsync.js

@@ -12,6 +12,18 @@ var start = Date.now();
 var err;
 var caught = false;
 
+// Verify that stderr is not accessed when a bad shell is used
+assert.throws(
+  function() { execSync('exit -1', {shell: 'bad_shell'}); },
+  /ENOENT/,
+  'execSync did not throw the expected exception!'
+);
+assert.throws(
+  function() { execFileSync('exit -1', {shell: 'bad_shell'}); },
+  /ENOENT/,
+  'execFileSync did not throw the expected exception!'
+);
+
 try {
   var cmd = `"${process.execPath}" -e "setTimeout(function(){}, ${SLEEP});"`;
   var ret = execSync(cmd, {timeout: TIMER});