make the PyUnicode_FSConverter cleanup set the decrefed argument to NULL (closes #25630)

Misc/NEWS

@@ -11,6 +11,9 @@ Release date: TBA
 Core and Builtins
 -----------------
 
+- Issue #25630: Fix a possible segfault during argument parsing in functions
+  that accept filesystem paths.
+
 - Issue #23564: Fixed a partially broken sanity check in the _posixsubprocess
   internals regarding how fds_to_pass were passed to the child.  The bug had
   no actual impact as subprocess.py already avoided it.

Objects/unicodeobject.c

@@ -3614,6 +3614,7 @@ PyUnicode_FSConverter(PyObject* arg, void* addr)
     void *data;
     if (arg == NULL) {
         Py_DECREF(*(PyObject**)addr);
+        *(PyObject**)addr = NULL;
         return 1;
     }
     if (PyBytes_Check(arg)) {