Guard against index error in prediction. (#6982)

* Remove `best_ntree_limit` from documents.

src/gbm/gbtree.cc

@@ -501,6 +501,7 @@ void GBTree::PredictBatch(DMatrix* p_fmat,
   uint32_t tree_begin, tree_end;
   std::tie(tree_begin, tree_end) =
       detail::LayerToTree(model_, tparam_, layer_begin, layer_end);
+  CHECK_LE(tree_end, model_.trees.size()) << "Invalid number of trees.";
   if (tree_end > tree_begin) {
     predictor->PredictBatch(p_fmat, out_preds, model_, tree_begin, tree_end);
   }