Fix XMP write and discarded return value.

[vrabaud]

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 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

[asmorkalov]

cc @vpisarev

[vrabaud]

This patch matters for 4.12.1 (if any). Otherwise, this code does not compile with no_discard (C++17) and a recent libavif.

[vpisarev]

@asmorkalov, I think, it should be merged, the sooner the better