stream: Emit error on stream object, not global

Apparently this function got abstracted out at some point, and 'this'
wasn't changed to the correct object.

lib/_stream_transform.js

@@ -89,7 +89,7 @@ function afterTransform(stream, er, data) {
   var cb = ts.writecb;
 
   if (!cb)
-    return this.emit('error', new Error('no writecb in Transform class'));
+    return stream.emit('error', new Error('no writecb in Transform class'));
 
   ts.writechunk = null;
   ts.writecb = null;