Fixed a segfault in the DCA decoder with corrupted streams.

It happens when the number of channels defined by DCAContext:acmod is lower
than DCAContext:prim_channels. In this case, dca_subsubframe() will call
qmf_32_subbands() using s->channel_order_tab[] entries equal to -1.

Originally committed as revision 22083 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/dca.c

@@ -1244,6 +1244,10 @@ static int dca_decode_frame(AVCodecContext * avctx,
         } else
             s->channel_order_tab = dca_channel_reorder_nolfe[s->amode];
 
+        if (s->prim_channels > 0 &&
+            s->channel_order_tab[s->prim_channels - 1] < 0)
+            return -1;
+
         if(avctx->request_channels == 2 && s->prim_channels > 2) {
             channels = 2;
             s->output = DCA_STEREO;