Seal clipboard buffers after copying data into them.

This is just a quick safety mechanism to ensure that nobody alters the
contents of a clipping after it's been set.

Ultimately this will be replaced by a more sophisticated SHM object.

WindowServer/WSClientConnection.cpp

@@ -336,6 +336,7 @@ void WSClientConnection::handle_request(WSAPIGetClipboardContentsRequest&)
         RetainPtr<SharedBuffer> shared_buffer = SharedBuffer::create(m_pid, WSClipboard::the().size());
         ASSERT(shared_buffer);
         memcpy(shared_buffer->data(), WSClipboard::the().data(), WSClipboard::the().size());
+        shared_buffer->seal();
         response.clipboard.shared_buffer_id = shared_buffer->shared_buffer_id();
         response.clipboard.contents_size = WSClipboard::the().size();