stream: prevent object map change in TransformState

TransformState has the writeencoding property that gets set on the
first _write. It is not declared when the transform state is initially
constructed and can cause a deopt.

PR-URL: nodejs#5032
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_stream_transform.js

@@ -58,6 +58,7 @@ function TransformState(stream) {
   this.transforming = false;
   this.writecb = null;
   this.writechunk = null;
+  this.writeencoding = null;
 }
 
 function afterTransform(stream, er, data) {