See issue #20121

The SIMD code in smooth.simd.hpp adds two pixel values before multiplying by weights, which leads to data overflow:

v_mul_expand(vx_load(src + j * cn) + vx_load(src + (n - 1 - j)*cn), vx_setall_u16((uint16_t) *((uint32_t*)(m + j))), v_add0, v_add1);

Both 3.4 and master branches are affected.

Test case is also added in this PR.

relates #18983

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