matroskadec: request a read buffer for the wav header

Solve an infiniloop.

CC: libav-stable@libav.org

libavformat/matroskadec.c

@@ -1496,7 +1496,7 @@ static int matroska_read_header(AVFormatContext *s)
                    && track->codec_priv.data != NULL) {
             int ret;
             ffio_init_context(&b, track->codec_priv.data, track->codec_priv.size,
-                          AVIO_FLAG_READ, NULL, NULL, NULL, NULL);
+                              0, NULL, NULL, NULL, NULL);
             ret = ff_get_wav_header(&b, st->codec, track->codec_priv.size);
             if (ret < 0)
                 return ret;