avformat/mvdec: fix reading number of audio channels

The number of audio channels is stored after the magic number
identifying the audio format.  Prior to this patch the code has been
reading it earlier, causing files with only one audio channel to be
handled incorrectly.

Reviewed-by: Peter Ross <pross@xvid.org>

libavformat/mvdec.c

@@ -340,8 +340,8 @@ static int mv_read_header(AVFormatContext *avctx)
             return AVERROR_INVALIDDATA;
         }
         avpriv_set_pts_info(ast, 33, 1, ast->codecpar->sample_rate);
-        if (set_channels(avctx, ast, avio_rb32(pb)) < 0)
-            return AVERROR_INVALIDDATA;
+
+        avio_skip(pb, 4);
 
         v = avio_rb32(pb);
         if (v == AUDIO_FORMAT_SIGNED) {
@@ -350,7 +350,11 @@ static int mv_read_header(AVFormatContext *avctx)
             avpriv_request_sample(avctx, "Audio compression (format %i)", v);
         }
 
-        avio_skip(pb, 12);
+        if (set_channels(avctx, ast, avio_rb32(pb)) < 0)
+            return AVERROR_INVALIDDATA;
+
+        avio_skip(pb, 8);
+
         var_read_metadata(avctx, "title", 0x80);
         var_read_metadata(avctx, "comment", 0x100);
         avio_skip(pb, 0x80);