4.x: Fix data corruption in WeChatQRCode::impl::decode

[elatkin]

Data corruption may lead to incorrect results if multiple QR found at image

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 another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the 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

allow_multiple_commits=1

[mshabunin]

Is it possible to add simple test case reproducing the issue? E.g. with some synthetic image or one of existing test images?

[mshabunin]

I suggest also changing method signature to use const vector<Mat>& candidate_points and for-loop to use const auto& point for consistency.