Ensuring the fragment isAdded before to continue with the AsyncTask.

[develric]

This PR is related to some consideration made during #11343 review (see here for more context)

With the following call path in ReaderPostListFragment:

LoadTagsTask -> onPostExecute -> onFilterCriteriasLoaded -> setCurrentFilter(mFilterListener.onRecallSelection())

it can happen that the onRecallSelection (who calls requireActivity) could be called when the fragment is not added so resulting in a crash. This PR tries to mitigate that possibility checking that the fragment is attached to activity in onPostExecute

To test

Was not able to trigger the issue so not fully sure how to test. At least smoke test the reader to check that we did not introduce issues.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[malinajirka]

LGTM, thanks!