subtitle not work when play rate is negative

Submitted by Haihua Hu `@JaredHu`

I use playbin to play a out of band subtitle, when I set play rate to -2, the subtitle will disappear and print serveral critical message:

(gplay-1.0:1586): GStreamer-CRITICAL **: gst_segment_to_running_time_full: assertion 'stop != -1' failed