src: fix -Wunused-result warning in e38bade

This patch fixes the warning introduced by the changes in e38bade.

Ref: https://github.com/nodejs/node/pull/6092
PR-URL: https://github.com/nodejs/node/pull/6276
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>

src/node.cc

@@ -3229,7 +3229,8 @@ void SetupProcessObject(Environment* env,
 
   // pre-set _events object for faster emit checks
   Local<Object> events_obj = Object::New(env->isolate());
-  events_obj->SetPrototype(env->context(), Null(env->isolate()));
+  maybe = events_obj->SetPrototype(env->context(), Null(env->isolate()));
+  CHECK(maybe.FromJust());
   process->Set(env->events_string(), events_obj);
 }