child_process: add fork/execFile arg validation

Validate fork/execFile arguments.

Fixes: https://github.com/nodejs/node/issues/2681
Refs: https://github.com/nodejs/node/pull/4508
PR-URL: https://github.com/nodejs/node/pull/7399
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/child_process.js

@@ -19,15 +19,20 @@ const ChildProcess = exports.ChildProcess = child_process.ChildProcess;
 exports.fork = function(modulePath /*, args, options*/) {
 
   // Get options and args arguments.
-  var options, args, execArgv;
-  if (Array.isArray(arguments[1])) {
-    args = arguments[1];
-    options = util._extend({}, arguments[2]);
-  } else if (arguments[1] && typeof arguments[1] !== 'object') {
-    throw new TypeError('Incorrect value of args option');
-  } else {
-    args = [];
-    options = util._extend({}, arguments[1]);
+  var execArgv;
+  var options = {};
+  var args = [];
+  var pos = 1;
+  if (Array.isArray(arguments[pos])) {
+    args = arguments[pos++];
+  }
+
+  if (arguments[pos] != null) {
+    if (typeof arguments[pos] !== 'object') {
+      throw new TypeError('Incorrect value of args option');
+    } else {
+      options = util._extend({}, arguments[pos++]);
+    }
   }
 
   // Prepare arguments for fork:
@@ -132,7 +137,7 @@ exports.execFile = function(file /*, args, options, callback*/) {
     callback = arguments[pos++];
   }
 
-  if (pos === 1 && arguments.length > 1) {
+  if (!callback && arguments[pos] != null) {
     throw new TypeError('Incorrect value of args option');
   }