Update color_cvt.hpp, fix a potential overflow bug in Lab2BGR func

[TracelessLe]

fix a potential overflow bug in cv::cuda::cvtColor(cv::COLOR_Lab2BGR) func, which need to clamp on B/G/R.

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• 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=Custom
buildworker:Custom=linux-4,linux-6
build_image:Custom=ubuntu-cuda:16.04
Xbuild_image:Custom=ubuntu-cuda:18.04

[alalek]

Did you run CUDA tests locally?

[TracelessLe]

Did you run CUDA tests locally?

Sure, I did local tests in my project and using the "opencv/build/bin/opencv_test_cudaimgproc" executable file to verify both. This PR aims to fix the potential overflow bug in cv::cuda::cvtColor(cv::COLOR_Lab2BGR) function, which may cause some abnormal color if not clamp the float B/G/R value to range (0.0, 1.0).

The abnormal situation is such as attached below, where channel Green is overflow from float -0.0...1 (a very small negative number) to uint 255:

Some reference codes use cpu to do this color conversion are listed here.

[alalek]

Thank you for contribution 👍