[3.12] gh-126108: Fix potential null pointer dereference in PySys_AddWarnOptionUnicode (GH-126118) (#129522)

gh-126108: Fix potential null pointer dereference in `PySys_AddWarnOptionUnicode` (GH-126118)
(cherry picked from commit fad36bf)

Co-authored-by: Valery Fedorenko <federicovalenso@gmail.com>

Author: miss-islington

Misc/NEWS.d/next/Security/2024-10-29-09-15-10.gh-issue-126108.eTIjHY.rst

@@ -0,0 +1 @@
+Fix a possible ``NULL`` pointer dereference in :c:func:`!PySys_AddWarnOptionUnicode`.

Python/sysmodule.c

@@ -2653,6 +2653,7 @@ PySys_ResetWarnOptions(void)
 static int
 _PySys_AddWarnOptionWithError(PyThreadState *tstate, PyObject *option)
 {
+    assert(tstate != NULL);
     PyObject *warnoptions = get_warnoptions(tstate);
     if (warnoptions == NULL) {
         return -1;
@@ -2667,11 +2668,11 @@ void
 PySys_AddWarnOptionUnicode(PyObject *option)
 {
     PyThreadState *tstate = _PyThreadState_GET();
+    _Py_EnsureTstateNotNULL(tstate);
+    assert(!_PyErr_Occurred(tstate));
     if (_PySys_AddWarnOptionWithError(tstate, option) < 0) {
         /* No return value, therefore clear error state if possible */
-        if (tstate) {
-            _PyErr_Clear(tstate);
-        }
+        _PyErr_Clear(tstate);
     }
 }