Use of alloca in vorbis_comment_add_tag

Dear Vorbis devs,

I'm Robert Kausch, author of fre:ac - free audio converter.

Please consider using _ogg_malloc/_ogg_free in place of alloca in 
vorbis_comment_add_tag. alloca will cause undefined behaviour/crashing 
when it causes a stack overflow which can easily happen when adding 
cover art in a METADATA_BLOCK_PICTURE comment.

I had a user trying to convert a FLAC file with a 2 MB embedded cover 
art that caused a crash in vorbis_comment_add_tag.

Thanks and best regards,
Robert

PS @ list moderator: I sent message before I joined the list a few days 
ago, but it was ignored. The mail should still be in the moderation 
queue; please disregard that one.

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Robert Kausch
[email protected]