mp3dec: fix reading the Xing tag

The quality scale field is only supposed to be present if the fourth bit
is set. In practice, lame always sets it, but other tools might not.

CC:libav-stable@libav.org

libavformat/mp3dec.c

@@ -36,6 +36,7 @@
 #define XING_FLAG_FRAMES 0x01
 #define XING_FLAG_SIZE   0x02
 #define XING_FLAG_TOC    0x04
+#define XING_FLAC_QSCALE 0x08
 
 #define XING_TOC_COUNT 100
 
@@ -165,7 +166,8 @@ static void mp3_parse_info_tag(AVFormatContext *s, AVStream *st,
                                        st->time_base));
 
     /* VBR quality */
-    avio_rb32(s->pb);
+    if (v & XING_FLAC_QSCALE)
+        avio_rb32(s->pb);
 
     /* Encoder short version string */
     memset(version, 0, sizeof(version));