Throw exception when calling method without instance (instead of segfault)

Fixes pmed#106

Author: jcmonnin

test/test_class.cpp

@@ -218,6 +218,10 @@ void test_class_()
 	check_eq("X::my_static_var", run_script<int>(context, "X.my_static_var"), 1);
 	check_eq("X::my_static_var after assign", run_script<int>(context, "X.my_static_var = 123; X.my_static_var"), 123);
 
+	check_ex<std::runtime_error>("call method with invalid instance", [&context]() {
+		run_script<int>(context, "x = new X(); f = x.fun1; f(1)");
+	});
+
 	check_eq("JSON.stringify(X)",
 		run_script<std::string>(context, "JSON.stringify({'obj': new X(10), 'arr': [new X(11), new X(12)] })"),
 		R"({"obj":{"key":"obj","var":10},"arr":[{"key":"0","var":11},{"key":"1","var":12}]})"

v8pp/function.hpp

@@ -136,9 +136,14 @@ invoke(v8::FunctionCallbackInfo<v8::Value> const& args, std::true_type /*is_memb
 
 	v8::Isolate* isolate = args.GetIsolate();
 	v8::Local<v8::Object> obj = args.This();
-	return call_from_v8<Traits, class_type, F>(
-		*class_<class_type, Traits>::unwrap_object(isolate, obj),
-		std::forward<F>(get_external_data<F>(args.Data())), args);
+	auto ptr = class_<class_type, Traits>::unwrap_object(isolate, obj);
+	if (ptr) {
+		return call_from_v8<Traits, class_type, F>(
+			*ptr,
+			std::forward<F>(get_external_data<F>(args.Data())), args);
+	} else {
+		throw std::runtime_error("method called on null instance");
+	}
 }
 
 template<typename Traits, typename F>