Fix skimage.measure.label segfault

[rfezzani]

Description

Fixes #5260.

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.

[jni]

Thanks @rfezzani! My initial instinct was to aim for putting the check in the Python function, but that still would leave us open to segfaults e.g. if someone internally used the Cython for some other reason. So, this is strictly better I reckon. 👍

[jni]

The optional deps test failure is from a new dask release and not related to this PR.

[grlee77]

Thanks for the quick fix, @rfezzani