LibWeb: Ignore MessagePort.start() on already-closed port

This matches other browsers, and fixes an assertion found by Domato.

Tests/LibWeb/Text/expected/Messaging/MessagePort-start-after-close.txt

@@ -0,0 +1 @@
+PASS (didn't crash)

Tests/LibWeb/Text/input/Messaging/MessagePort-start-after-close.html

@@ -0,0 +1,10 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        let channel = new MessageChannel();
+        let port = channel.port2;
+        port.close();
+        port.start();
+        println("PASS (didn't crash)");
+    });
+</script>

Userland/Libraries/LibWeb/HTML/MessagePort.cpp

@@ -364,6 +364,9 @@ void MessagePort::post_message_task_steps(SerializedTransferRecord& serialize_wi
 // https://html.spec.whatwg.org/multipage/web-messaging.html#dom-messageport-start
 void MessagePort::start()
 {
+    if (!is_entangled())
+        return;
+
     VERIFY(m_socket);
     VERIFY(m_fd_passing_socket);