stutter: fix closure() on Fin-acceptance

From a report by Anton Pirogov.

* NEWS: Mention the bug.
* spot/twaalgos/stutter.cc: Fix it.
* tests/core/stutter-tgba.test: Test it.
* THANKS: Add Anton.
10 jobs for next in 123 minutes and 58 seconds (queued for 2 seconds)
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failed arch-clang #3749

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passed arch-gcc-glibcxxdebug #3750

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passed debian-gcc-snapshot #3748

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passed debian-stable-gcc #3746

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passed debian-unstable-gcc-coverage #3747

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passed debpkg-stable #3753

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passed debpkg-unstable #3754

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passed mingw-shared #3751

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passed mingw-static #3752

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  Publish
skipped publish-unstable #3755
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