vaapivideobufferpool: validate returned meta
vaapivideobufferpool: validate returned meta

Validate if the meta returned by gst_buffer_get_vaapi_video_meta() in
the acquired buffer is not null.

This situation should be very "pathological", but still it is better
be safe since that meta might be used later to create a new dma
buffer.

GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi