Added an is_valid function to FuncRef so script can check if it is safe to call it.

core/func_ref.h

@@ -46,6 +46,7 @@ public:
 	Variant call_func(const Variant **p_args, int p_argcount, Variant::CallError &r_error);
 	void set_instance(Object *p_obj);
 	void set_function(const StringName &p_func);
+	bool is_valid() const;
 	FuncRef();
 };