systemd-python: fix segfault on double close

TODO

@@ -570,6 +570,8 @@ Features:
    - allow reading of only select fields in systemd.journal._reader.Reader
    - export sd_journal_test_cursor in systemd.journal._reader.Reader
    - export sd_journal_get_usage in systemd.journal._reader.Reader
+   - add systemd.journal._reader._Reader.closed attribute (it can
+     be just "return self->j != NULL")
    - figure out a simple way to wait for journal events in a way that
      works with ^C
 

src/python-systemd/_reader.c

@@ -171,6 +171,7 @@ PyDoc_STRVAR(Reader_close__doc__,
 static PyObject* Reader_close(Reader *self, PyObject *args)
 {
     sd_journal_close(self->j);
+    self->j = NULL;
     Py_RETURN_NONE;
 }