LibIPC: Disable Notifier before closing socket

Because we're closing a file descriptor, we need to disable any
Notifier that is using it so that the EventLoop does not use invalid
file descriptors.

Fixes #3508

Libraries/LibIPC/Connection.h

@@ -115,6 +115,7 @@ public:
 
     void shutdown()
     {
+        m_notifier->close();
         m_socket->close();
         die();
     }