ib_cursor_open_index_using_name() for index name that doesn't exist causes segfault

[Stewart Smith]

 *ib_crsr = NULL;

 if (index_id > 0) {
  err = ib_create_cursor(
   ib_crsr, table, index_id, cursor->prebuilt->trx);
 }

 if (ib_crsr != NULL) {
  const ib_cursor_t* cursor;

  cursor = *(ib_cursor_t**) ib_crsr;

********** if (cursor->prebuilt->index == NULL) {
   err = ib_cursor_close(*ib_crsr);
   ut_a(err == DB_SUCCESS);
   *ib_crsr = NULL;
  }
 }

of course, dereferencing NULL doesn't work so well.