Fix Laplacian matrix size bug in random_walker

[grlee77]

Description

closes #5034 using the suggestion from @morgan-bc in that issue (thanks!). The max size as used here looks like the maximum possible value for the unlabeled_indices array that is used later in indexing the rows and columns.

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.

[rfezzani]

Thank you @grlee77, perfect 😉

[emmanuelle]

CI failure does not seem related to this PR, merging. Thanks for the fix @grlee77 !