[Bug 709814] [examples/overlay] avoid to unref sink if not found. Also fix logic to find a sink in one of the example.

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=709814
  GStreamer | gst-plugins-base | git

--- Comment #1 from Julien Isorce <julien.isorce at gmail.com> 2013-10-10 13:18:44 UTC ---
Created an attachment (id=256904)
 View: https://bugzilla.gnome.org/attachment.cgi?id=256904
 Review: https://bugzilla.gnome.org/review?bug=709814&attachment=256904

handle the case when xvimagesink is not found

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