Fix monotone constraints on CPU. (#9122)

src/tree/hist/evaluate_splits.h

@@ -412,6 +412,7 @@ class HistEvaluator {
     tree_evaluator_.AddSplit(candidate.nid, left_child, right_child,
                              tree[candidate.nid].SplitIndex(), left_weight,
                              right_weight);
+    evaluator = tree_evaluator_.GetEvaluator();
 
     snode_.resize(tree.GetNodes().size());
     snode_.at(left_child).stats = candidate.split.left_sum;

src/tree/split_evaluator.h

@@ -49,6 +49,8 @@ class TreeEvaluator {
       monotone_.HostVector().resize(n_features, 0);
       has_constraint_ = false;
     } else {
+      CHECK_LE(p.monotone_constraints.size(), n_features)
+          << "The size of monotone constraint should be less or equal to the number of features.";
       monotone_.HostVector() = p.monotone_constraints;
       monotone_.HostVector().resize(n_features, 0);
       // Initialised to some small size, can grow if needed