fs: fix typo in fs.readFile of lying size=0 stat

lib/fs.js

@@ -167,7 +167,7 @@ fs.readFile = function(path, encoding_) {
     fs.close(fd, function(er) {
       if (size === 0) {
         // collected the data into the buffers list.
-        buffer = Buffer.concat(buffer.length, pos);
+        buffer = Buffer.concat(buffers, pos);
       }
 
       if (encoding) buffer = buffer.toString(encoding);