src: fixing debug process stuck

The debug process running "node debug a.js" will be stuck when the
script ends. This is because the debug handler has been unrefed.
We shouldn't unref the debug handler to avoid this problem.

src/node.cc

@@ -3652,7 +3652,6 @@ void Init(int* argc,
   uv_async_init(uv_default_loop(),
                 &dispatch_debug_messages_async,
                 DispatchDebugMessagesAsyncCallback);
-  uv_unref(reinterpret_cast<uv_handle_t*>(&dispatch_debug_messages_async));
 
 #if defined(NODE_V8_OPTIONS)
   // Should come before the call to V8::SetFlagsFromCommandLine()
@@ -3959,8 +3958,11 @@ static void StartNodeInstance(void* arg) {
     env->set_trace_sync_io(trace_sync_io);
 
     // Enable debugger
-    if (instance_data->use_debug_agent())
+    if (instance_data->use_debug_agent()) {
       EnableDebug(env);
+    } else {
+      uv_unref(reinterpret_cast<uv_handle_t*>(&dispatch_debug_messages_async));
+    }
 
     {
       SealHandleScope seal(isolate);