Hi, OpenCV Team

This PR tries to fix #12818 on branch 3.4.

Though labled as won't fix in existing issues, there are still some programs using cvSaveImage( ). I just happen to migrate a project with many OpenCV C API calls to C++ API calls, comparing two implementations results, and encounter opencv 3.4 latest version (3.4.13)'s cvSaveImage( ) still crash on Windows platform. Thus I would like to share my solution.

Pull Request Readiness Checklist

• 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

Apart from this PR's fix method, people could also write a simple function as alternative to the bug cvSaveImage:

void mySaveImage(const char* filename, IplImage* dst)
{
    cv::Mat img = cv::cvarrToMat(dst);
    cv::imwrite(filename, img);
}