ac3dec: apply dynamic range compression to correct channels in dual-mono mode.

Originally committed as revision 20533 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/ac3dec.c

@@ -1167,7 +1167,7 @@ static int decode_audio_block(AC3DecodeContext *s, int blk)
     for(ch=1; ch<=s->channels; ch++) {
         float gain = s->mul_bias / 4194304.0f;
         if(s->channel_mode == AC3_CHMODE_DUALMONO) {
-            gain *= s->dynamic_range[ch-1];
+            gain *= s->dynamic_range[2-ch];
         } else {
             gain *= s->dynamic_range[0];
         }