Validates VisualScript.add_node input node

(cherry picked from commit 0ca38ffe766de1debfd44211db2c8b9265baffba)

modules/visual_script/visual_script.cpp

@@ -294,6 +294,7 @@ void VisualScript::_node_ports_changed(int p_id) {
 void VisualScript::add_node(const StringName &p_func, int p_id, const Ref<VisualScriptNode> &p_node, const Point2 &p_pos) {
 	ERR_FAIL_COND(instances.size());
 	ERR_FAIL_COND(!functions.has(p_func));
+	ERR_FAIL_COND(p_node.is_null());
 
 	for (Map<StringName, Function>::Element *E = functions.front(); E; E = E->next()) {
 		ERR_FAIL_COND(E->get().nodes.has(p_id)); //id can exist only one in script, even for different functions