Fixed some functions possibly vulnerable for concurrent calls

After interestingly reading the report in the mailing list (http://mail.kde.org/pipermail/taglib-devel/2013-October/002561.html), I had a brief look at the code. Though I couldn't locate the problem, I found some functions which are possibly vulnerable for concurrent calls.

I'm not sure that this can resolve the problem, but it can make TagLib a little more robust.