Fix warning in clang version 10.0.0-4ubuntu1

warning: loop variable 'nh' of type 'const ade::Handle<ade::Node>' creates a copy from type 'const ade::Handle<ade::Node>' [-Wrange-loop-construct]

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

force_builders=ARMv7,Custom
build_image:Custom=centos:7
buildworker:Custom=linux-1