[useTranslation] Avoid setting the new t function if the component is unmounted.

[zozoens31]

Once the translations are loaded from backend, the hook sets its t function without checking that the component is still mounted, which induces an error from react-dom.

Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in a useEffect cleanup function.

I've updated the code to keep the isMounted pattern to be consistent with the previous version, but I'll gladly change it for a promise cancellation if you find it more appropriate.

[coveralls]

Coverage increased (+0.01%) to 95.327% when pulling ee21d84 on bayesimpact:cyrille-no-callback into c764b02 on i18next:master.

[sprilukin]

+1 for the issue.

[jamuhl]

Looks valid...never run into this myself - but thinking about it the case is valid. Will deeper check this when back from holidays (this weekend, early next week) and merge this into master.

[jamuhl]

sorry forgot about this one...published in react-i18next@11.3.4