src: ensure `v8::Eternal` is empty before setting it

src: ensure `v8::Eternal` is empty before setting it · nodejs/node@8a5325d

Original file line number	Diff line number	Diff line change
`@@ -847,6 +847,7 @@ void Environment::set_process_exit_handler(`
`847`	`847`	`return PropertyName##_.Get(isolate_); \`
`848`	`848`	`} \`
`849`	`849`	`inline void IsolateData::set_##PropertyName(v8::Local<TypeName> value) { \`
	`850`	`+CHECK(PropertyName##_.IsEmpty()); \`
`850`	`851`	`PropertyName##_.Set(isolate_, value); \`
`851`	`852`	`}`
`852`	`853`	`PER_ISOLATE_TEMPLATE_PROPERTIES(V)`