avcodec/dxva2_h264: fix slice offset in long slice struct after ca2f19b9

The GetBitContext now includes the NAL header, which offsets the slice by
one byte, which needs to be accounted for here.

libavcodec/dxva2_h264.c

@@ -228,7 +228,7 @@ static void fill_slice_long(AVCodecContext *avctx, DXVA_Slice_H264_Long *slice,
 
     slice->first_mb_in_slice     = (sl->mb_y >> FIELD_OR_MBAFF_PICTURE(h)) * h->mb_width + sl->mb_x;
     slice->NumMbsForSlice        = 0; /* XXX it is set once we have all slices */
-    slice->BitOffsetToSliceData  = get_bits_count(&sl->gb);
+    slice->BitOffsetToSliceData  = get_bits_count(&sl->gb) - 8;
     slice->slice_type            = ff_h264_get_slice_type(sl);
     if (sl->slice_type_fixed)
         slice->slice_type += 5;