Without libid3tag, we were trying to skip the ID3 frame (since
0.15.2).  Its length however was not calculated at all, we were just
dropping everything from the current input buffer.  This lead to the
first few seconds of the file being skipped.  This patch attempts to
calculate the ID3v2 frame size with the formula from:

 http://www.id3.org/id3v2.4.0-structure 3.1 and 6.2