fix(wechat_code): Modify `isnan` for compatibility with -ffast_math.

[Konano]

fix #3150 Merge with https://github.com/opencv/opencv/pull/23881

Reference: https://stackoverflow.com/questions/7263404/mingw32-stdisnan-with-ffast-math

Pull Request Readiness Checklist

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

• [x] I agree to contribute to the project under Apache 2 License.
• [x] 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
• [x] The PR is proposed to the proper branch
• [x] There is a reference to the original bug report and related work
• [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable Patch to opencv_extra has the same branch name.
• [x] The feature is well documented and sample code can be built with the project CMake

[asmorkalov]

@Konano Friendly reminder.