avcodec/mpeg4video_parser: Don't set write-only current_picture_ptr

It is unused by ff_mpeg4_decode_picture_header() (unsurprisingly given
that when decoding this function is called before the context has been
initialized).

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

libavcodec/mpeg4video_parser.c

@@ -89,7 +89,6 @@ static int mpeg4_decode_header(AVCodecParserContext *s1, AVCodecContext *avctx,
     int ret;
 
     s->avctx               = avctx;
-    s->current_picture_ptr = &s->current_picture;
 
     if (avctx->extradata_size && pc->first_picture) {
         init_get_bits(gb, avctx->extradata, avctx->extradata_size * 8);