Fix random_walker regression

[rfezzani]

Description

Fix #4770.

Regression was introduced in 8f76f0f.

This PR

• fixes random_walker segmentation when labels contains isolated area
• adds test_isolated_area

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[sciunto]

Looks perfect! thanks @rfezzani

Might be worth backporting it.

[sciunto]

@scikit-image/core We need a second review on this bugfix (easy to read).

[stefanv]

If the test failures are unrelated, go ahead and merge.

[sciunto]

They are. i'm merging. Thanks @stefanv

[sciunto]

@meeseeksdev backport to v0.17.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout v0.17.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 6ea4052408e5dc969d612a0d26255e65d2efd104

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #4771: Fix random_walker regression'

4. Push to a named branch :

git push YOURFORK v0.17.x:auto-backport-of-pr-4771-on-v0.17.x

5. Create a PR against branch v0.17.x, I would have named this PR:

"Backport PR #4771 on branch v0.17.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.

[rfezzani]

🎉 Thank you @sciunto ;)