[3.12] gh-109496: Detect Py_DECREF() after dealloc in debug mode (GH-109539) (#109545)

gh-109496: Detect Py_DECREF() after dealloc in debug mode (GH-109539)

On a Python built in debug mode, Py_DECREF() now calls
_Py_NegativeRefcount() if the object is a dangling pointer to
deallocated memory: memory filled with 0xDD "dead byte" by the debug
hook on memory allocators. The fix is to check the reference count
*before* checking for _Py_IsImmortal().

Add test_decref_freed_object() to test_capi.test_misc.
(cherry picked from commit 0bb0d88e2d4e300946e399e088e2ff60de2ccf8c)

Co-authored-by: Victor Stinner <vstinner@python.org>