message: Prevent null dereference read in `SpecializeType()`
  For some adversarial protos, the attribute for a key might not exist.
diff: @@ -22,6 +22,7 @@ limitations under the License.
  #include "tensorflow/core/framework/op_def.pb.h"
  #include "tensorflow/core/framework/types.h"
  #include "tensorflow/core/platform/statusor.h"
  +#include "tensorflow/core/protobuf/error_codes.pb.h"
  namespace tensorflow {
  @@ -93,7 +94,11 @@ StatusOr<FullTypeDef> SpecializeType(const AttrSlice& attrs,
  auto* arg = t->mutable_args(i);
  if (arg->type_id() == TFT_VAR) {
  const auto* attr = attrs.Find(arg->s());
  - DCHECK(attr != nullptr);
  + if (attr == nullptr) {
  + return Status(
  + error::INVALID_ARGUMENT,
  + absl::StrCat("Could not find an attribute for key ", arg->s()));
  + }
  if (attr->value_case() == AttrValue::kList) {
  const auto& attr_list = attr->list();
  arg->set_type_id(TFT_PRODUCT);
  