decoder: AV1: Fix a static analysis problem of update_state().
decoder: AV1: Fix a static analysis problem of update_state().

No need to check the picture pointer after we have already dereferenced it.

Fix: #298
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/408>

GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi