bpo-32869: Fix incorrect dst buffer size for MultiByteToWideChar (pythonGH-5739)

This function expects the destination buffer size to be given
in wide characters, not bytes.
(cherry picked from commit b3b4a9d)

Co-authored-by: Alexey Izbyshev <[email protected]>

Author: miss-islington

Diff for: Python/fileutils.c

@@ -1185,7 +1185,8 @@ _Py_fopen_obj(PyObject *path, const char *mode)
     if (wpath == NULL)
         return NULL;
 
-    usize = MultiByteToWideChar(CP_ACP, 0, mode, -1, wmode, sizeof(wmode));
+    usize = MultiByteToWideChar(CP_ACP, 0, mode, -1,
+                                wmode, Py_ARRAY_LENGTH(wmode));
     if (usize == 0) {
         PyErr_SetFromWindowsErr(0);
         return NULL;