about 1 out of every 10 runs of gst-python-s testsuite/test_pad.py, the
preroll_queued drops from 1 to 0 during the state commit.

I changed the g_assert to give an ERROR:

    /* FIXME: a pad probe could have made us lose the buffer, according
     * to one of the python tests */
    if (length == 0) {
      GST_ERROR_OBJECT (basesink,
          "preroll_queued dropped from 1 to 0 while committing state change");
    }
    g_assert (length <= 1);
  }


but this probably needs a better fix.