ac3dec: fix outptr increment.

Fixes decoding regression
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ac3dec.c

@@ -1402,9 +1402,8 @@ static int ac3_decode_frame(AVCodecContext * avctx, void *data,
                 memcpy(((float*)s->frame.data[ch]) + AC3_BLOCK_SIZE*blk, output[ch], 1024);
         for (ch = 0; ch < s->out_channels; ch++) {
             output[ch] = s->outptr[channel_map[ch]];
-        }
-        for (ch = 0; ch < s->channels; ch++) {
-            s->outptr[ch] += AC3_BLOCK_SIZE;
+            if (!ch || channel_map[ch])
+                s->outptr[channel_map[ch]] += AC3_BLOCK_SIZE;
         }
     }