au: move skipping of unused data to before parameter validation

Also do not unnecessarily skip 0 bytes.

libavformat/au.c

@@ -81,6 +81,11 @@ static int au_read_header(AVFormatContext *s)
     rate = avio_rb32(pb);
     channels = avio_rb32(pb);
 
+    if (size > 24) {
+        /* skip unused data */
+        avio_skip(pb, size - 24);
+    }
+
     codec = ff_codec_get_id(codec_au_tags, id);
 
     if (codec == AV_CODEC_ID_NONE) {
@@ -99,11 +104,6 @@ static int au_read_header(AVFormatContext *s)
         return AVERROR_INVALIDDATA;
     }
 
-    if (size >= 24) {
-        /* skip unused data */
-        avio_skip(pb, size - 24);
-    }
-
     /* now we are ready: build format streams */
     st = avformat_new_stream(s, NULL);
     if (!st)