gh-99578: Fix refleak in _imp.create_builtin() (GH-99642)

Fix a reference bug in _imp.create_builtin() after the creation of
the first sub-interpreter for modules "builtins" and "sys".
(cherry picked from commit cb2ef8b2ac)

Co-authored-by: Victor Stinner <vstinner@python.org>

Python/import.c

@@ -1006,7 +1006,8 @@ create_builtin(PyThreadState *tstate, PyObject *name, PyObject *spec)
         if (_PyUnicode_EqualToASCIIString(name, p->name)) {
             if (p->initfunc == NULL) {
                 /* Cannot re-init internal module ("sys" or "builtins") */
-                return PyImport_AddModuleObject(name);
+                mod = PyImport_AddModuleObject(name);
+                return Py_XNewRef(mod);
             }
 
             mod = (*p->initfunc)();