dv: Fix null pointer dereference due to ach=0

Fixes part2 of CVE-2011-3929

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Reviewed-by: Roman Shaposhnik <roman@shaposhnik.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/dv.c

@@ -384,6 +384,7 @@ int avpriv_dv_produce_packet(DVDemuxContext *c, AVPacket *pkt,
        c->audio_pkt[i].pts  = c->abytes * 30000*8 / c->ast[i]->codec->bit_rate;
        ppcm[i] = c->audio_buf[i];
     }
+    if (c->ach)
         dv_extract_audio(buf, ppcm, c->sys);
 
     /* We work with 720p frames split in half, thus even frames have