[BUGFIX] Avoid unacceptable slug changes on locked slugs

[dkd-kaehm]

EXT:sluggi always(on each pages change) rewrites the slug with path segments to the last path segment, even the slug is locked and not changed by editor. Also from /virtual-parent-slug/my-test-page to /my-test-page. This change prevents overrides on slugs at least on "locked" slugs.

Note: This is a partial fix only and not locked slugs will be still overridden.

Relates: #98