Refactor fs.open parameter handling

Improvements:
* Removes an unnecessary variable
* Avoids having two variables with the same name
* Avoids re-declaring an existing parameter
* Removes an unnecessary ternary operator
* Avoid an inline short-circuit expression for greater clarity.

lib/fs.js

@@ -185,12 +185,14 @@ function modeNum(m, def) {
 }
 
 fs.open = function(path, flags, mode, callback) {
-  var callback_ = arguments[arguments.length - 1];
-  var callback = (typeof(callback_) == 'function' ? callback_ : null);
+  callback = arguments[arguments.length - 1];
+  if (typeof(callback) !== 'function') {
+    callback = noop;
+  }
 
   mode = modeNum(mode, '0666');
 
-  binding.open(path, stringToFlags(flags), mode, callback || noop);
+  binding.open(path, stringToFlags(flags), mode, callback);
 };
 
 fs.openSync = function(path, flags, mode) {