basesink: Unset the last buffer list if we only got a buffer
basesink: Unset the last buffer list if we only got a buffer

Also remember any preroll buffer list.

jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,krichter722/gstreamer,krichter722/gstreamer,centricular/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,centricular/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,krichter722/gstreamer