[C++] Check categorical feature bin number for warning causes Segmentfault when there are features being ignored

[shiyu1994]

Description

In #4448, a check for the number of bins in categorical features is introduced. However, in

LightGBM/src/io/dataset_loader.cpp

Line 1488 in b462d0a

if (bin_mappers[i]->bin_type() == BinType::CategoricalBin && bin_mappers[i]->num_bin() > max_bin_for_this_feature) {

the nullable of bin_mappers[i] is not check, which can cause segment fault when there exist features being ignored.

Environment info

LightGBM version or commit hash:
Latest master

[github-actions]

This issue has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.