Make stream copy of ac3 from riff based formats work.

Originally committed as revision 13408 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffmpeg.c

@@ -1677,6 +1677,8 @@ static int av_encode(AVFormatContext **output_files,
                 codec->block_align= icodec->block_align;
                 if(codec->block_align == 1 && codec->codec_id == CODEC_ID_MP3)
                     codec->block_align= 0;
+                if(codec->codec_id == CODEC_ID_AC3)
+                    codec->block_align= 0;
                 break;
             case CODEC_TYPE_VIDEO:
                 if(using_vhook) {