dxva2_h264: fix slice offset in long slice structs

Fixes Ticket #4384
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dxva2_h264.c

@@ -222,7 +222,7 @@ 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(&h->gb);
+    slice->BitOffsetToSliceData  = get_bits_count(&sl->gb);
     slice->slice_type            = ff_h264_get_slice_type(sl);
     if (sl->slice_type_fixed)
         slice->slice_type += 5;