vaapidecode: Don't crash if a buffer outlives the decoder
vaapidecode: Don't crash if a buffer outlives the decoder

Sometimes, for example, when switching video streams but keeping
the same sink, the surface will be released after the decoder is
stopped and replaced. This caused a crash because the release
callback was called on an invalid pointer.
The patch adding an additional reference to the decoder object in the buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=745189

Signed-off-by: Olivier Crete <86e32882ba3951d68bd2d06702dd09a9d6fd15fe@collabora.com>
Signed-off-by: Sreerenj Balachandran <8526e100bdc7a12d85af109c9f42b86756e50e67@intel.com>

gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi