Fix memory leak in cudacodec

[cudawarped]

In #3098 I carelessly introduced a memory leak, this fix should address that issue.

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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 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]

@VadimLevin I've added the changes you suggested.

[asmorkalov]

Tested with Ubuntu 18.04 and Cuda 10.2