dnn: fix index access

modules/dnn/src/dnn.cpp

@@ -2759,8 +2759,12 @@ struct Net::Impl : public detail::NetImplBase
                         it->second.out.empty())
                 {
                     getLayerShapesRecursively(layerId, inOutShapes);
+                    it = inOutShapes.find(layerId);
+                    CV_Assert(it != inOutShapes.end());
                 }
-                const MatShape& shape = inOutShapes[layerId].out[inputLayerIds[i].oid];
+                const int out_port = inputLayerIds[i].oid;
+                CV_CheckLT(out_port, (int)it->second.out.size(), "");
+                const MatShape& shape = it->second.out[out_port];
                 layerShapes.in.push_back(shape);
             }
         }