GH-94644: fix test_curses ref leak (GH-94647)

(cherry picked from commit 277f55cb04)

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>

Modules/_cursesmodule.c

@@ -383,6 +383,7 @@ PyCurses_ConvertToString(PyCursesWindowObject *win, PyObject *obj,
             return 0;
         /* check for embedded null bytes */
         if (PyBytes_AsStringAndSize(*bytes, &str, NULL) < 0) {
+            Py_CLEAR(*bytes);
             return 0;
         }
         return 1;