stream: only check options once in Duplex ctor

This commit updates the Duplex constructor adding an if statement
checking if options is undefined, and removes the check from the
following three if statements.

PR-URL: #20353
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/_stream_duplex.js

@@ -50,17 +50,19 @@ function Duplex(options) {
 
   Readable.call(this, options);
   Writable.call(this, options);
+  this.allowHalfOpen = true;
 
-  if (options && options.readable === false)
-    this.readable = false;
+  if (options) {
+    if (options.readable === false)
+      this.readable = false;
 
-  if (options && options.writable === false)
-    this.writable = false;
+    if (options.writable === false)
+      this.writable = false;
 
-  this.allowHalfOpen = true;
-  if (options && options.allowHalfOpen === false) {
-    this.allowHalfOpen = false;
-    this.once('end', onend);
+    if (options.allowHalfOpen === false) {
+      this.allowHalfOpen = false;
+      this.once('end', onend);
+    }
   }
 }