vaapiav1dec: NULL dereference issue

This issue was discovered via static analysis.

In av1_decoder_update_state, the picture variable in the for loop on line 979 is checked for NULL after it has already been dereferenced on previous line.

  for (i = 0; i < GST_AV1_NUM_REF_FRAMES; i++) {
    if ((picture->frame_header.refresh_frame_flags >> i) & 1) {
      if (picture) {
        GST_LOG ("reference frame %p to ref slot:%d", picture, i);
        gst_vaapi_picture_replace (&priv->ref_frames[i], picture);
      } else {
        GST_ERROR ("we miss some reference frame for ref slot:%d", i);
        gst_vaapi_picture_replace (&priv->ref_frames[i], NULL);
      }
    }
  }

Please fix the logic.

cc: @He_Junyan