stream: remove useless if test in transform

PR-URL: nodejs#4617
Reviewed-By: Roman Reiss <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Brian White <[email protected]>

Author: zoubin

lib/_stream_transform.js

@@ -75,8 +75,7 @@ function afterTransform(stream, er, data) {
   if (data !== null && data !== undefined)
     stream.push(data);
 
-  if (cb)
-    cb(er);
+  cb(er);
 
   var rs = stream._readableState;
   rs.reading = false;