gh-97728: Argument Clinic: Fix uninitialized variable in the Py_UNICODE converter (GH-97729)

It affects function os.system() on Windows and Windows-specific modules
winreg, _winapi, _overlapped, and _msi.

Author: serhiy-storchaka

Lib/test/clinic.test

@@ -1803,12 +1803,12 @@ static PyObject *
 test_Py_UNICODE_converter(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
 {
     PyObject *return_value = NULL;
-    const Py_UNICODE *a;
-    const Py_UNICODE *b;
-    const Py_UNICODE *c;
-    const Py_UNICODE *d;
+    const Py_UNICODE *a = NULL;
+    const Py_UNICODE *b = NULL;
+    const Py_UNICODE *c = NULL;
+    const Py_UNICODE *d = NULL;
     Py_ssize_t d_length;
-    const Py_UNICODE *e;
+    const Py_UNICODE *e = NULL;
     Py_ssize_t e_length;
 
     if (!_PyArg_ParseStack(args, nargs, "O&O&O&u#Z#:test_Py_UNICODE_converter",
@@ -1833,7 +1833,7 @@ test_Py_UNICODE_converter_impl(PyObject *module, const Py_UNICODE *a,
                                const Py_UNICODE *b, const Py_UNICODE *c,
                                const Py_UNICODE *d, Py_ssize_t d_length,
                                const Py_UNICODE *e, Py_ssize_t e_length)
-/*[clinic end generated code: output=4d426808cdbb3ea3 input=064a3b68ad7f04b0]*/
+/*[clinic end generated code: output=9f34a249b3071fdd input=064a3b68ad7f04b0]*/
 
 
 /*[clinic input]

Misc/NEWS.d/next/Windows/2022-10-02-11-59-23.gh-issue-97728.dIdlPE.rst

@@ -0,0 +1,3 @@
+Fix possible crashes caused by the use of uninitialized variables when pass
+invalid arguments in :func:`os.system` on Windows and in Windows-specific
+modules (like ``winreg``).

Modules/clinic/_winapi.c.h

@@ -3586,6 +3586,7 @@ def converter_init(self, *, accept={str}, zeroes=False):
                 self.converter = '_PyUnicode_WideCharString_Opt_Converter'
             else:
                 fail("Py_UNICODE_converter: illegal 'accept' argument " + repr(accept))
+        self.c_default = "NULL"
 
     def cleanup(self):
         if not self.length: