vaapidecode: handle decode-only frames.
vaapidecode: handle decode-only frames.

Decode-only frames may not have a valid surface proxy. So, simply discard
them gracefully, i.e. don't create meta data information. GstVideoDecoder
base class will properly handle this case and won't try to push any buffer
to downstream elements.

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