There doubts in these two commits (bug 749607):

commit d6255be9398f5d6803c0997446a4b89c10beab26
Author: Víctor Manuel Jáquez Leal <victorx.jaquez@intel.com>
Date:   Wed May 27 10:49:56 2015 +0200

    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.

commit f1a60ec6a2d9523c10713a43871df22ddc68d720
Author: Jan Schmidt <jan@centricular.com>
Date:   Wed Dec 17 00:41:10 2014 +1100

    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.

In commit d6255be9

(In reply to Gwenole Beauchesne from comment #13)
> It would be better to know why slice_hdr or unit is NULL. Committing this is
> just hiding the core issue more...

In commit f1a60ec6 (from discussion in https://github.com/thaytan/gstreamer-vaapi/commit/b84e65ada15f9396b8c9ab507c1cb5f10c3f3914)

>+    if (!is_valid_state(decoder, GST_MPEG_VIDEO_STATE_VALID_PIC_HEADERS))
>+        return GST_VAAPI_DECODER_STATUS_SUCCESS;
>+
> This part should not be necessary. This is already tracked at decode time. I 
> would just push the two other hunks.

This bug is to verify if those changeset are correct or not.