dnn/NGraph: added nullptr checks

modules/dnn/src/dnn.cpp

@@ -2023,6 +2023,7 @@ struct Net::Impl : public detail::NetImplBase
                         Ptr<BackendNode> inpNode = inpLd.backendNodes[preferableBackend];
                         if (!inpNode.empty()) {
                             Ptr<InfEngineNgraphNode> ieNode = inpNode.dynamicCast<InfEngineNgraphNode>();
+                            CV_Assert(!ieNode.empty());
                             ieNode->net->setUnconnectedNodes(ieNode);
                         }
                     }
@@ -2067,6 +2068,7 @@ struct Net::Impl : public detail::NetImplBase
                         int cons_inp = cons->oid;
                         Ptr<NgraphBackendWrapper> inpWrapper = inpLd.outputBlobsWrappers[cons_inp].
                                                                      dynamicCast<NgraphBackendWrapper>();
+                        CV_Assert(!inpWrapper.empty());
                         auto iter = std::find(inputNames.begin(), inputNames.end(),
                                               inpWrapper->dataPtr->getName());
                         if (iter == inputNames.end()) {