tls: remove sharedCreds in Server constructor

This commit removes the var sharedCreds which is just reassigned to
this._sharedCreds in the following line.

PR-URL: #20491
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/_tls_wrap.js

@@ -871,7 +871,7 @@ function Server(options, listener) {
   // Handle option defaults:
   this.setOptions(options);
 
-  var sharedCreds = tls.createSecureContext({
+  this._sharedCreds = tls.createSecureContext({
     pfx: this.pfx,
     key: this.key,
     passphrase: this.passphrase,
@@ -887,7 +887,6 @@ function Server(options, listener) {
     crl: this.crl,
     sessionIdContext: this.sessionIdContext
   });
-  this._sharedCreds = sharedCreds;
 
   this[kHandshakeTimeout] = options.handshakeTimeout || (120 * 1000);
   this[kSNICallback] = options.SNICallback;
@@ -898,11 +897,11 @@ function Server(options, listener) {
   }
 
   if (this.sessionTimeout) {
-    sharedCreds.context.setSessionTimeout(this.sessionTimeout);
+    this._sharedCreds.context.setSessionTimeout(this.sessionTimeout);
   }
 
   if (this.ticketKeys) {
-    sharedCreds.context.setTicketKeys(this.ticketKeys);
+    this._sharedCreds.context.setTicketKeys(this.ticketKeys);
   }
 
   // constructor call