stream: avoid possible slow path w UInt8Array

A chunk validity checks verifie if a chunk is a UInt8Array.
We should defer it as it might be very expensive in older Node.js
platforms.

lib/_stream_writable.js

@@ -248,7 +248,7 @@ function validChunk(stream, state, chunk, cb) {
 Writable.prototype.write = function(chunk, encoding, cb) {
   var state = this._writableState;
   var ret = false;
-  var isBuf = Stream._isUint8Array(chunk) && !state.objectMode;
+  var isBuf = !state.objectMode && Stream._isUint8Array(chunk);
 
   if (isBuf && Object.getPrototypeOf(chunk) !== Buffer.prototype) {
     chunk = Stream._uint8ArrayToBuffer(chunk);