[3.12] gh-105673: Fix uninitialized warning in sysmodule.c (GH-105674) (#105675)

In sys_add_xoption(), 'value' may be uninitialized for some error paths.
(cherry picked from commit a8d69fe)

Co-authored-by: Nikita Sobolev <[email protected]>

Author: miss-islington

Python/sysmodule.c

@@ -3371,7 +3371,7 @@ _PySys_InitCore(PyThreadState *tstate, PyObject *sysdict)
 static int
 sys_add_xoption(PyObject *opts, const wchar_t *s)
 {
-    PyObject *name, *value;
+    PyObject *name, *value = NULL;
 
     const wchar_t *name_end = wcschr(s, L'=');
     if (!name_end) {