message: Bounds-check node ID before getting it's name
  When the edge is either a frame enter or exit edge then
  DescribeCycle() would segfault.
diff: @@ -35,6 +35,7 @@ limitations under the License.
  #include "tensorflow/core/framework/types.h"
  #include "tensorflow/core/graph/algorithm.h"
  #include "tensorflow/core/graph/control_flow.h"
  +#include "tensorflow/core/kernels/bounds_check.h"
  #include "tensorflow/core/lib/strings/strcat.h"
  #include "tensorflow/core/public/version.h"
  @@ -432,6 +433,9 @@ string DescribeCycle(const GraphCycles& cycles, const Graph& graph, int src,
  }
  auto node_name = [&cycles, &graph](int node_id) {
  + if (!FastBoundsCheck(node_id, graph.num_node_ids())) {
  + return string("(null)");
  + }
  auto* node = graph.FindNodeId(node_id);
  if (node == nullptr) {
  return string("(null)");
  