stream: remove useless check

lib/_stream_readable.js

@@ -360,8 +360,7 @@ function chunkInvalid(state, chunk) {
       'string' !== typeof chunk &&
       chunk !== null &&
       chunk !== undefined &&
-      !state.objectMode &&
+      !state.objectMode) {
-      !er) {
     er = new TypeError('Invalid non-string/buffer chunk');
   }
   return er;