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.
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failed arch-clang #3741

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passed arch-gcc-glibcxxdebug #3742

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passed debian-gcc-snapshot #3740

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passed debian-stable-gcc #3738

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passed debian-unstable-gcc-coverage #3739

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88.9%
passed debpkg-stable #3745

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passed mingw-shared #3743

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passed mingw-static #3744

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