coverity scan defects fixed

- coverity scan defects handled

```Defect(s) Reported-by: Coverity Scan
Showing 4 of 4 defect(s)


** CID 60490:  Unchecked dynamic_cast  (FORWARD_NULL)
/home/buildslave/coverity_analysis/build/src/shogun/multiclass/tree/C45ClassifierTree.cpp: 483 in shogun::CC45ClassifierTree::apply_multiclass_from_current_node(shogun::CDenseFeatures<double> *, shogun::CTreeMachineNode<shogun::C45TreeNodeData> *)()

** CID 60491:  Unchecked dynamic_cast  (FORWARD_NULL)
/home/buildslave/coverity_analysis/build/src/shogun/multiclass/tree/C45ClassifierTree.cpp: 499 in shogun::CC45ClassifierTree::apply_multiclass_from_current_node(shogun::CDenseFeatures<double> *, shogun::CTreeMachineNode<shogun::C45TreeNodeData> *)()

** CID 60492:  Unchecked dynamic_cast  (FORWARD_NULL)
/home/buildslave/coverity_analysis/build/src/shogun/multiclass/tree/C45ClassifierTree.cpp: 459 in shogun::CC45ClassifierTree::apply_multiclass_from_current_node(shogun::CDenseFeatures<double> *, shogun::CTreeMachineNode<shogun::C45TreeNodeData> *)()

** CID 60493:  Unchecked dynamic_cast  (FORWARD_NULL)
/home/buildslave/coverity_analysis/build/src/shogun/multiclass/tree/C45ClassifierTree.cpp: 138 in shogun::CC45ClassifierTree::C45train(shogun::CFeatures *, shogun::SGVector<double>, shogun::CMulticlassLabels *, shogun::SGVector<int>, int)()


________________________________________________________________________________________________________
*** CID 60490:  Unchecked dynamic_cast  (FORWARD_NULL)
/home/buildslave/coverity_analysis/build/src/shogun/multiclass/tree/C45ClassifierTree.cpp: 483 in shogun::CC45ClassifierTree::apply_multiclass_from_current_node(shogun::CDenseFeatures<double> *, shogun::CTreeMachineNode<shogun::C45TreeNodeData> *)()
477                             }
478                             // if not nominal attribute check if greater or less than threshold
479                             else
480                             {
481                                     node_t* left_child=dynamic_cast<node_t*>(children->get_element(0));
482                                     node_t* right_child=dynamic_cast<node_t*>(children->get_element(1));
>>>     CID 60490:  Unchecked dynamic_cast  (FORWARD_NULL)
>>>     Dereferencing null pointer "left_child".
483                                     if (left_child->data.transit_if_feature_value>=sample[node->data.attribute_id])
484                                     {
485                                             SG_UNREF(node);
486                                             node=left_child;
487                                             SG_REF(left_child)
488

________________________________________________________________________________________________________
*** CID 60491:  Unchecked dynamic_cast  (FORWARD_NULL)
/home/buildslave/coverity_analysis/build/src/shogun/multiclass/tree/C45ClassifierTree.cpp: 499 in shogun::CC45ClassifierTree::apply_multiclass_from_current_node(shogun::CDenseFeatures<double> *, shogun::CTreeMachineNode<shogun::C45TreeNodeData> *)()
493                                     {
494                                             SG_UNREF(node);
495                                             node=right_child;
496                                             SG_REF(right_child)
497
498                                             SG_UNREF(children);
>>>     CID 60491:  Unchecked dynamic_cast  (FORWARD_NULL)
>>>     Passing null pointer "node" to "get_children", which dereferences it. (The dereference happens because this is a virtual function call.)
499                                             children=node->get_children();
500                                     }
501
502                                     SG_UNREF(left_child);
503                                     SG_UNREF(right_child);
504                             }

________________________________________________________________________________________________________
*** CID 60492:  Unchecked dynamic_cast  (FORWARD_NULL)
/home/buildslave/coverity_analysis/build/src/shogun/multiclass/tree/C45ClassifierTree.cpp: 459 in shogun::CC45ClassifierTree::apply_multiclass_from_current_node(shogun::CDenseFeatures<double> *, shogun::CTreeMachineNode<shogun::C45TreeNodeData> *)()
453                             // if nominal attribute check for equality
454                             if (m_nominal[node->data.attribute_id])
455                             {
456                                     for (int32_t j=0; j<children->get_num_elements(); j++)
457                                     {
458                                             node_t* child=dynamic_cast<node_t*>(children->get_element(j));
>>>     CID 60492:  Unchecked dynamic_cast  (FORWARD_NULL)
>>>     Dereferencing null pointer "child".
459                                             if (child->data.transit_if_feature_value==sample[node->data.attribute_id])
460                                             {
461                                                     flag=true;
462
463                                                     SG_UNREF(node);
464                                                     node=child;

________________________________________________________________________________________________________
*** CID 60493:  Unchecked dynamic_cast  (FORWARD_NULL)
/home/buildslave/coverity_analysis/build/src/shogun/multiclass/tree/C45ClassifierTree.cpp: 138 in shogun::CC45ClassifierTree::C45train(shogun::CFeatures *, shogun::SGVector<double>, shogun::CMulticlassLabels *, shogun::SGVector<int>, int)()
132
133     CTreeMachineNode<C45TreeNodeData>* CC45ClassifierTree::C45train(CFeatures* data, SGVector<float64_t> weights,
134             CMulticlassLabels* class_labels, SGVector<int32_t> feature_id_vector, int32_t level)
135     {
136             node_t* node=new node_t();
137             CDenseFeatures<float64_t>* feats=dynamic_cast<CDenseFeatures<float64_t>*>(data);
>>>     CID 60493:  Unchecked dynamic_cast  (FORWARD_NULL)
>>>     Passing null pointer "feats" to "get_num_vectors", which dereferences it. (The dereference happens because this is a virtual function call.)
138             int32_t num_vecs=feats->get_num_vectors();
139
140             // set class_label for the node as the mode of occurring multiclass labels
141             SGVector<float64_t> labels=class_labels->get_labels_copy();
142             labels.qsort();
143```