fix index out of range bug.

src/io/dense_nbits_bin.hpp

@@ -206,7 +206,8 @@ public:
   void LoadFromMemory(const void* memory, const std::vector<data_size_t>& local_used_indices) override {
     const uint8_t* mem_data = reinterpret_cast<const uint8_t*>(memory);
     if (!local_used_indices.empty()) {
-      for (int i = 0; i < num_data_; i += 2) {
+      const data_size_t rest = num_data_ & 1;
+      for (int i = 0; i < num_data_ - rest; i += 2) {
         // get old bins
         data_size_t idx = local_used_indices[i];
         const auto bin1 = static_cast<uint8_t>((mem_data[idx >> 1] >> ((idx & 1) << 2)) & 0xf);
@@ -216,7 +217,7 @@ public:
         const int i1 = i >> 1;
         data_[i1] = (bin1 | (bin2 << 4));
       }
-      if ((num_data_ & 1) == 1) {
+      if (rest) {
         data_size_t idx = local_used_indices[num_data_ - 1];
         data_[num_data_ / 2 + 1] = (mem_data[idx >> 1] >> ((idx & 1) << 2)) & 0xf;
       }
@@ -229,7 +230,8 @@ public:
 
   void CopySubset(const Bin* full_bin, const data_size_t* used_indices, data_size_t num_used_indices) override {
     auto other_bin = reinterpret_cast<const Dense4bitsBin*>(full_bin);
-    for (int i = 0; i < num_used_indices; i += 2) {
+    const data_size_t rest = num_used_indices & 1;
+    for (int i = 0; i < num_used_indices - rest; i += 2) {
       data_size_t idx = used_indices[i];
       const auto bin1 = static_cast<uint8_t>((other_bin->data_[idx >> 1] >> ((idx & 1) << 2)) & 0xf);
       idx = used_indices[i + 1];
@@ -237,7 +239,7 @@ public:
       const int i1 = i >> 1;
       data_[i1] = (bin1 | (bin2 << 4));
     }
-    if ((num_used_indices & 1) == 1) {
+    if (rest) {
       data_size_t idx = used_indices[num_used_indices - 1];
       data_[num_used_indices / 2 + 1] = (other_bin->data_[idx >> 1] >> ((idx & 1) << 2)) & 0xf;
     }