mp3dec: fix truncating packet error message while decoding a complete MP3.

The MP3 demuxer split the data in packets of 1024B which are later split
in MP3 frames by the MPEG audio parser. The last read is "truncated",
but this should not raise any error.

Solution-by: Michael Niedermayer

libavformat/mp3dec.c

@@ -149,6 +149,7 @@ static int mp3_read_header(AVFormatContext *s)
     // lcm of all mp3 sample rates
     avpriv_set_pts_info(st, 64, 1, 14112000);
 
+    s->pb->maxsize = -1;
     off = avio_tell(s->pb);
 
     if (!av_dict_get(s->metadata, "", NULL, AV_DICT_IGNORE_SUFFIX))