avformat/nutdec: fix packet end clearing

The code was buggy, using the wrong variable, also it missed the case
where the packet become smaller due to sidedata/metadata being extracted
which left a few bytes uninitialized

Fixes use of uninitialized memory
Fixed: msan_uninit-mem_7f6abbe44530_6838_mewmew_vorbis_ssa.nut
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/nutdec.c

@@ -1011,8 +1011,8 @@ static int decode_frame(NUTContext *nut, AVPacket *pkt, int frame_code)
     if (ret != size) {
         if (ret < 0)
             return ret;
-        av_shrink_packet(pkt, nut->header_len[header_idx] + size);
     }
+    av_shrink_packet(pkt, nut->header_len[header_idx] + ret);
 
     pkt->stream_index = stream_id;
     if (stc->last_flags & FLAG_KEY)