avformat/nutdec: Don't shrink packet size manually

It is unnecessary because an av_shrink_packet() a few lines below
will set the size; furthermore, it is actually harmful, because
av_shrink_packet() does nothing in case the size already matches,
so that the packet's padding is not correctly zeroed.

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

libavformat/nutdec.c

@@ -1132,7 +1132,6 @@ static int decode_frame(NUTContext *nut, AVPacket *pkt, int frame_code)
         }
         sm_size = avio_tell(bc) - pkt->pos;
         size      -= sm_size;
-        pkt->size -= sm_size;
     }
 
     ret = avio_read(bc, pkt->data + nut->header_len[header_idx], size);