Taglib segfaults - Null String

Recently I faced up to some crashes due to metadata extraction... mainly when I'm trying to read a lot of embedded_covers at the same time. I realized that Mixxx is getting the info directly from the tag and trying to convert to QString and it was also the cause of the crashes.

total nitpick:  can you say if (tstring ==  null) { return QString() } since that's the special case?  I just like the look of it more that way :)