[MP] Fix remote net ID cleanup

Synchronizers for spawned nodes were not correctly keeping track of the
net ID assigned by the remote, preventing the replication from
performing the proper cleanup.

This resulted in errors being thrown when sync messages were received
after despawn (which is possible due to their unreliable nature).

modules/multiplayer/multiplayer_synchronizer.cpp

@@ -49,11 +49,11 @@ void MultiplayerSynchronizer::_stop() {
 	}
 #endif
 	root_node_cache = ObjectID();
-	reset();
 	Node *node = is_inside_tree() ? get_node_or_null(root_path) : nullptr;
 	if (node) {
 		get_multiplayer()->object_configuration_remove(node, this);
 	}
+	reset();
 }
 
 void MultiplayerSynchronizer::_start() {

modules/multiplayer/scene_replication_interface.cpp

@@ -249,6 +249,7 @@ Error SceneReplicationInterface::on_replication_start(Object *p_obj, Variant p_c
 		uint32_t net_id = pending_sync_net_ids[0];
 		pending_sync_net_ids.pop_front();
 		peers_info[pending_spawn_remote].recv_sync_ids[net_id] = sync->get_instance_id();
+		sync->set_net_id(net_id);
 
 		// Try to apply spawn state (before ready).
 		if (pending_buffer_size > 0) {