[RABIT] fix rabit in local mode

include/rabit/engine.h

@@ -41,6 +41,8 @@ class IEngine {
   typedef void (ReduceFunction) (const void *src,
                                  void *dst, int count,
                                  const MPI::Datatype &dtype);
+  /*! \brief virtual destructor */
+  virtual ~IEngine() {}
   /*!
    * \brief performs in-place Allreduce, on sendrecvbuf
    *        this function is NOT thread-safe

src/allreduce_robust.cc

@@ -79,7 +79,7 @@ void AllreduceRobust::Allreduce(void *sendrecvbuf_,
                                 PreprocFunction prepare_fun,
                                 void *prepare_arg) {
   // skip action in single node
-  if (world_size == 1) {
+  if (world_size == 1 || world_size == -1) {
     if (prepare_fun != NULL) prepare_fun(prepare_arg);
     return;
   }