src: fix handle leak in UDPWrap::Instantiate()

Create a handle scope before performing a check that creates a handle,
otherwise the handle is leaked into the handle scope of the caller.

PR-URL: https://github.com/nodejs/node/pull/7711
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

src/udp_wrap.cc

@@ -436,9 +436,9 @@ void UDPWrap::OnRecv(uv_udp_t* handle,
 
 
 Local<Object> UDPWrap::Instantiate(Environment* env, AsyncWrap* parent) {
+  EscapableHandleScope scope(env->isolate());
   // If this assert fires then Initialize hasn't been called yet.
   CHECK_EQ(env->udp_constructor_function().IsEmpty(), false);
-  EscapableHandleScope scope(env->isolate());
   Local<Value> ptr = External::New(env->isolate(), parent);
   return scope.Escape(env->udp_constructor_function()
       ->NewInstance(env->context(), 1, &ptr).ToLocalChecked());