[Bug 791638] playbin3: Fix accessing invalid index in GstStream when received select-stream event

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=791638

--- Comment #1 from Gabby Park <di85.park at gmail.com> ---
My test scenario is as below:
1) Track info of Test Stream
- 1 Video
- 2 Audio
- 1 Subtitle

2) Sent stream-select event info
- index 0 video
- index 1 audio
- no subtitle

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