LibWebRTC TCPConnection might receive packets while its port is nullptr

https://bugs.webkit.org/show_bug.cgi?id=260705
rdar://113531400

Reviewed by Jean-Yves Avenard.

According logs, we have a nullptr crash in Connection::OnReadPacket when calling Port::GetStunMessage.
The current explanation is this one:
1. The connection is live and connected to the socket (which means it is a TCPConnection).
2. The connection's port is dead, which can happen if Port::DestroyConnectionAsync is called.

To prevent the nullptr crash, we add a nullptr check in TCPConnection::OnReadPacket.

* Source/ThirdParty/libwebrtc/Source/webrtc/p2p/base/tcp_port.cc:

Canonical link: https://commits.webkit.org/267275@main

Source/ThirdParty/libwebrtc/Source/webrtc/p2p/base/tcp_port.cc

@@ -564,6 +564,14 @@ void TCPConnection::OnReadPacket(rtc::AsyncPacketSocket* socket,
                                  const rtc::SocketAddress& remote_addr,
                                  const int64_t& packet_time_us) {
   RTC_DCHECK_EQ(socket, socket_.get());
+
+#if defined(WEBRTC_WEBKIT_BUILD)
+  if (!port()) {
+    RTC_LOG(LS_WARNING) << "TCPConnection: Port has been deleted.";
+    return;
+  }
+#endif
+
   Connection::OnReadPacket(data, size, packet_time_us);
 }