Fix variable leak.

Fixes issue 461

lib/fs.js

@@ -196,7 +196,7 @@ fs.readSync = function (fd, buffer, offset, length, position) {
   if (!Buffer.isBuffer(buffer)) {
     // legacy string interface (fd, length, position, encoding, callback)
     legacy = true;
-    encoding = arguments[3];
+    var encoding = arguments[3];
     position = arguments[2];
     length = arguments[1];
     buffer = new Buffer(length);