Fix potential overflow in linear trees (#5395)

* Fix potential overflow in linear trees

* simplify
Co-authored-by: James Lamb <jaylamb20@gmail.com>

src/treelearner/linear_tree_learner.cpp

@@ -281,7 +281,7 @@ void LinearTreeLearner::CalculateLinear(Tree* tree, bool is_refit, const score_t
       float g = static_cast<float>(gradients[i]);
       int j = 0;
       for (int feat1 = 0; feat1 < num_feat + 1; ++feat1) {
-        float f1_val = curr_row[feat1];
+        double f1_val = static_cast<double>(curr_row[feat1]);
         XTg_by_thread_[tid][leaf_num][feat1] += f1_val * g;
         f1_val *= h;
         for (int feat2 = feat1; feat2 < num_feat + 1; ++feat2) {