stream: simplify writable's validChunk()

This commit simplifies validChunk() by removing an unnecessary
intermediate variable.

PR-URL: #20696
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Jackson Tian <shyvo1987@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_stream_writable.js

@@ -250,7 +250,6 @@ function writeAfterEnd(stream, cb) {
 // mode the stream is in. Currently this means that `null` is never accepted
 // and undefined/non-string values are only allowed in object mode.
 function validChunk(stream, state, chunk, cb) {
-  var valid = true;
   var er;
 
   if (chunk === null) {
@@ -261,9 +260,9 @@ function validChunk(stream, state, chunk, cb) {
   if (er) {
     stream.emit('error', er);
     process.nextTick(cb, er);
-    valid = false;
+    return false;
   }
-  return valid;
+  return true;
 }
 
 Writable.prototype.write = function(chunk, encoding, cb) {