set alac channels from extradata, fix alac mono in m4a

Originally committed as revision 16212 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mov.c

@@ -1033,8 +1033,10 @@ static int mov_read_stsd(MOVContext *c, ByteIOContext *pb, MOVAtom atom)
         st->codec->block_align = sc->bytes_per_frame;
         break;
     case CODEC_ID_ALAC:
-        if (st->codec->extradata_size == 36)
+        if (st->codec->extradata_size == 36) {
             st->codec->frame_size = AV_RB32((st->codec->extradata+12));
+            st->codec->channels   = AV_RB8  (st->codec->extradata+21);
+        }
         break;
     default:
         break;