bugfix: #1855 - PHP errors in APE if invalid values entered in MP3gain fields

http://www.getid3.org/phpBB3/viewtopic.php?t=1855
If invalid values are entered into APEtag fields used by MP3gain (e.g.
MP3GAIN_UNDO) then PHP errors could be shown.
getID3 now checks for correct format before parsing, and registers
warning if invalid data is found.