add missing cudaFree

[incubus-ank]

I found missing memory cudaFree in cv::cuda::FAST_Impl::detectAsync()

I created an issue, but did not receive a response.
#3994

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 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

[cudawarped]

@incubus-ank Would it be more performant to move the allocation into the constructor and free into the destructor of FAST_Impl to save the allocation on every invocatoin of FAST_Impl::detectAsync?

[incubus-ank]

@incubus-ank Would it be more performant to move the allocation into the constructor and free into the destructor of FAST_Impl to save the allocation on every invocatoin of FAST_Impl::detectAsync?

Yes, I think it's a good idea.

I'm not a pro at working with CUDA. Should I define/remove copy and move constructors to avoid funny situations and create a better interface?

[cudawarped]

I would say no because this class can only be created with the factory method cv::cuda::FastFeatureDetector::create and accessed through the resulting smart pointer, @asmorkalov keep me honest here?

Ideally I would suggest using a GpuMat member variable for d_counter which would handle the memory management for you but that would involve altering the type from unsigned int to int and I not sure if this has any implications.

[cudawarped]

LGTM! 👍