dv: Fix null pointer dereference due to ach=0

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>
Signed-off-by: Alex Converse <alex.converse@gmail.com>
(cherry picked from commit 5a396bb3a6)
Signed-off-by: Reinhard Tartler <siretart@tauware.de>
(cherry picked from commit 44e182d41e)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>
(cherry picked from commit b46141b0d1)

Conflicts:

	libavformat/dv.c

libavformat/dv.c

@@ -341,7 +341,8 @@ int 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];
     }
-    dv_extract_audio(buf, ppcm, c->sys);
+    if (c->ach)
+        dv_extract_audio(buf, ppcm, c->sys);
     c->abytes += size;
 
     /* We work with 720p frames split in half, thus even frames have