mpeg2: avoid crash when seeking with debug logs
mpeg2: avoid crash when seeking with debug logs

Move down the debug message when the state of the decoder is verified
so the slice header is not NULL.

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