Check for flatbuffer loading failures

Will report "Failed to build model from buffer" if it's invalid flatbuffer.

PiperOrigin-RevId: 691554844

src/builtin-adapter/direct_flatbuffer_to_json_graph_convert.cc

@@ -837,6 +837,9 @@ absl::StatusOr<std::string> ConvertFlatbufferDirectlyToJson(
   std::unique_ptr<FlatBufferModelAbslError> model_ptr =
       FlatBufferModelAbslError::VerifyAndBuildFromBuffer(
           model_content.data(), model_content.length());
+  if (model_ptr == nullptr) {
+    return absl::InvalidArgumentError("Failed to build model from buffer.");
+  }
 
   mlir::MLIRContext mlir_context;
   mlir::DialectRegistry registry;