fix bug in initialization of routing

include/rabit/engine.h

@@ -242,8 +242,10 @@ class ReduceHandle {
   static int TypeSize(const MPI::Datatype &dtype);
 
  protected:
-  // handle data field
+  // handle function field
   void *handle_;
+  // reduce function of the reducer
+  IEngine::ReduceFunction *redfunc_;
   // handle to the type field
   void *htype_;
   // the created type in 4 bytes

include/rabit/rabit-inl.h

@@ -201,7 +201,7 @@ inline void ReducerFunc_(const void *src_, void *dst_, int len_, const MPI::Data
   const char *psrc = reinterpret_cast<const char*>(src_);
   char *pdst = reinterpret_cast<char*>(dst_);
   DType tdst, tsrc;
-  for (size_t i = 0; i < len_; ++i) {
+  for (int i = 0; i < len_; ++i) {
     // use memcpy to avoid alignment issue
     std::memcpy(&tdst, pdst + i * kUnit, sizeof(tdst));
     std::memcpy(&tsrc, psrc + i * kUnit, sizeof(tsrc));