Would you better like to use
if ($data->link = JFilterOutput::stringURLSafe($data->link))
instead of
if (mb_check_encoding($data->link, 'ASCII'))

I think that would make more sense :)

Changed, please test again

Hmm...
if ($data->link = JFilterOutput::stringURLSafe($data->link))
will always be true since it's an assignment. I think you want to compare the strings like this:
if ($data->link == JFilterOutput::stringURLSafe($data->link))

I just went with forcing the issue :P infograf768#7

hmm, does not work
Moving back to previous code

@wilsonge
I have another solution with a regex,
Please test again

Latest patch seems to work. Validator validates.

When "All is well — The Travis CI build passed" is listed, do we need additional testing?

My last test was with 3331, and the below is with 3320:

WITHOUT PATCH 3320

With SEF enabled, URLs like the below cause RSS to fail:

• index.php/article-category-list/73-look-i-have-à-in-the-title

With SEF disabled, URLs don't include the alias, and RSS passes:

• index.php?option=com_content&view=article&id=73&catid=19&Itemid=260

WITH PATCH 3320

• Everything looks good to go

When "All is well — The Travis CI build passed" is listed, do we need additional testing?

Travis only checks if the unit tests pass. And there aren't unit tests for everything. Even if we have full coverage, there could still be errors which go in undetected by Travis.
So yes, manual testing is still needed and probably always will be needed.