lavf/id3v2: seek back to previous offset if header size is not matched

Makes ff_id3v2_read reset stream position at the end of ID3 data if the
header size is not matched (caused by an EOF for example).

Current behaviour (without the patch):
    filesize = 400
    id3 data size = 399
    file offset after ff_id3v2_read is 400 instead of 399

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>