ges-pipeline: Don't unref buffer obtained from a GstSample
ges-pipeline: Don't unref buffer obtained from a GstSample

https://bugzilla.gnome.org/show_bug.cgi?id=707914

cfoch/gst-editing-services,cfoch/gst-editing-services,cfoch/ges,cfoch/ges,cfoch/ges,cfoch/ges,cfoch/gst-editing-services,cfoch/gst-editing-services