dv: Fix out of array read

Fixes part of CVE-2011-3936

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/dv.c

@@ -35,6 +35,7 @@
 #include "libavutil/intreadwrite.h"
 #include "libavutil/mathematics.h"
 #include "dv.h"
+#include "libavutil/avassert.h"
 
 struct DVDemuxContext {
     const DVprofile*  sys;    /* Current DV profile. E.g.: 525/60, 625/50 */
@@ -130,15 +131,19 @@ static int dv_extract_audio(uint8_t* frame, uint8_t* ppcm[4],
     /* We work with 720p frames split in half, thus even frames have
      * channels 0,1 and odd 2,3. */
     ipcm = (sys->height == 720 && !(frame[1] & 0x0C)) ? 2 : 0;
-    pcm  = ppcm[ipcm++];
 
     /* for each DIF channel */
     for (chan = 0; chan < sys->n_difchan; chan++) {
+        av_assert0(ipcm<4);
+        pcm = ppcm[ipcm++];
+        if (!pcm)
+            break;
         /* for each DIF segment */
         for (i = 0; i < sys->difseg_size; i++) {
             frame += 6 * 80; /* skip DIF segment header */
             if (quant == 1 && i == half_ch) {
                 /* next stereo channel (12bit mode only) */
+                av_assert0(ipcm<4);
                 pcm = ppcm[ipcm++];
                 if (!pcm)
                     break;
@@ -183,9 +188,6 @@ static int dv_extract_audio(uint8_t* frame, uint8_t* ppcm[4],
         }
 
         /* next stereo channel (50Mbps and 100Mbps only) */
-        pcm = ppcm[ipcm++];
-        if (!pcm)
-            break;
     }
 
     return size;