Fix bug in SLIC superpixels with enforce_connectivity=True and start_label > 0

[grlee77]

Description

closes #6240

Prevent possible 0 label in output when start label > 0 for SLIC superpixels. This could occur when running "_enforce_label_connectivity_cython" because the adjacent variable started with value 0, but it seems like it must start with current_new_label (or start_label) to avoid potentially assigning a value outside the label range.

This is only really an issue at the upper edge/corner of the image where it is possible that adjacent never gets assigned because there are not yet any adjacent labels assigned to pull from.

Checklist

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[rfezzani]

Thank you @grlee77, LGTM 😉

[grlee77]

@meeseeksdev backport to v0.19.x

[mkcor]

Suggested change

	"""Tests the fix for a bug producing a label < start_label (gh-6240).
	"""Test the fix for a bug producing a label < start_label (gh-6240).

Sorry for coming late! Thanks for adding this test 😃