fix bug when splitting sparse bin with passed num_data=0 (#68)

src/io/sparse_bin.hpp

@@ -59,6 +59,8 @@ public:
 
   data_size_t Split(unsigned int threshold, data_size_t* data_indices, data_size_t num_data,
                          data_size_t* lte_indices, data_size_t* gt_indices) const override {
+    // not need to split
+    if (num_data <= 0) { return 0; }
     const auto fast_pair = fast_index_[(data_indices[0]) >> fast_index_shift_];
     data_size_t j = fast_pair.first;
     data_size_t cur_pos = fast_pair.second;