Use official comment size limit for vorbis tags

[Etenil]

The official vorbis size for comments is 2^32 bytes, and sadly I happen to have some files that make use of more than 64kB of comment space (somehow the tag editor stuffed a base64-encoded image in there).

An argument could be made that instead of expanding the maximum comment size, we could instead print a warning and create an empty comment; I can look into doing that if it's seen as more elegant.

Another point could be that those files should be edited; but technically speaking they do comply with the vorbis spec and I don't think listen.el should require users to know how to edit vorbis tags to listen to their music :-/.

Let me know if there's anything I should change.