node: fix leaking Context handle

The call to node::Environment::GetCurrent(Isolate*) makes the call to
v8::Isolate::GetCurrentContext(). Doing so creates a new handle that
bubbled to the v8::SealHandleScope().

PR-URL: nodejs/node#3945
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: James Snell <jasnell@gmail.com>

src/node.cc

@@ -3177,6 +3177,7 @@ static void EnableDebug(Environment* env) {
 
 // Called from the main thread.
 static void DispatchDebugMessagesAsyncCallback(uv_async_t* handle) {
+  HandleScope scope(node_isolate);
   if (debugger_running == false) {
     fprintf(stderr, "Starting debugger agent.\n");