avcodec/mpegvideo_enc: Don't update current_picture unnecessarily

current_picture is not changed after frame_start() at all
and it therefore does not need to be updated (i.e. copied to the
slice thread contexts) a second time.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/mpegvideo_enc.c

@@ -251,7 +251,6 @@ static void update_duplicate_context_after_me(MpegEncContext *dst,
 {
 #define COPY(a) dst->a= src->a
     COPY(pict_type);
-    COPY(current_picture);
     COPY(f_code);
     COPY(b_code);
     COPY(qscale);