mpeg2: Avoid crashes and warnings on re-opened decoder after a seek
mpeg2: Avoid crashes and warnings on re-opened decoder after a seek

Reset state and add some checks for safe state to avoid a crash and
a warning after the decoder is destroyed/recreated during a seek.

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