network-stack: Fix TcpConnection use-after-free

When lwIP notifies us about the PCB being closed or an error, we must
clear its user data so any future callbacks don't end up using the
TcpConnection after it's gone.

src/fruity/network-stack.vala

@@ -499,6 +499,11 @@ namespace Frida.Fruity {
 				});
 			}
 
+			private void detach_from_pcb () {
+				pcb.set_user_data (null);
+				pcb = null;
+			}
+
 			private void on_connect () {
 				lock (state)
 					tx_space_available = pcb.query_available_send_buffer_space ();
@@ -516,7 +521,7 @@ namespace Frida.Fruity {
 
 			private void on_recv (owned LWIP.PacketBuffer? pbuf, LWIP.ErrorCode err) {
 				if (pbuf == null) {
-					pcb = null;
+					detach_from_pcb ();
 					schedule_on_frida_thread (() => {
 						_state = CLOSED;
 						update_events ();
@@ -539,7 +544,7 @@ namespace Frida.Fruity {
 			}
 
 			private void on_error (LWIP.ErrorCode err) {
-				pcb = null;
+				detach_from_pcb ();
 				schedule_on_frida_thread (() => {
 					_state = CLOSED;
 					update_events ();