[issue38070] visit_decref(): add an assertion to check that the object is not freed

[STINNER Victor]

Change by STINNER Victor <vstinner at python.org>:


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pull_requests: +16217
pull_request: https://github.com/python/cpython/pull/16631

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38070>
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