Bugfix: Failed to execute 'update' on 'MediaKeySession

[hongfeih-es]

This PR will...

Fix several uncaught drm exceptions when call load/unload frequently.

Why is this Pull Request needed?

keySession.update is called in a callback. And it will raise exception when trying to update keys which belong to closed key session:

Uncaught (in promise) DOMException: Failed to execute 'update' on 'MediaKeySession': The session is already closed.
    at https://hls-js.netlify.app/dist/hls.js:6259:18
    at EMEController._onLicenseRequestReadyStageChange (https://hls-js.netlify.app/dist/hls.js:6457:11)

Are there any points in the code the reviewer needs to double check?

No

Resolves issues:

#4576

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[itsjamie]

Thank you!