If an M4A file contains trailing garbage (e.g. an APE tag at the
end of the file), it was considered invalid since [e21640b].
It is important to make sure that atoms which affect modifications
written by TagLib are valid, otherwise the file might be damaged.
However, in cases where invalid atoms do not affect modifications
(which are only done inside the root level "moov" and "moof"
containers), they can be safely ignored.
This commit allows trailing garbage in M4A files as it is also
accepted in MP3, WAV and DSF files.
491322d1ba