vorbisdec: don't put invalid bitrate values into the taglist

vorbisdec: don't put invalid bitrate values into the taglist

Bitrates are stored as 32-bit signed integers in the vorbis
identification headers, but seem to be read incorrectly,
namely as unsigned 32-bit integers, into the vorbis structure
members which are of type long, which makes our check for
values <= 0 fail with files that put -1 in there for unset
values.