decoder: h265: Fix offset calculation in codec_data parsing
decoder: h265: Fix offset calculation in codec_data parsing

https://bugzilla.gnome.org/show_bug.cgi?id=762922

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