SoundSource - checking if taglig::string is null

Taglib strings can be NULL and using it could cause some segfaults, so in this case it will return a QString()
SouundSource::toQString() - Cosmetic fix, changing return

@kain88-de:
I don't know if this is in the coding guidelines but usually in mixxx we exit function early only when an error occurs. The normal everything went ok exist is always at the end. In this case tstring being null is the error
cosmetic fix - soundsource::toQString() - remove extra white space
Merge branch 'master' of https://github.com/mixxxdj/mixxx into bug_taglib
Merge pull request #296 from cardinot/bug_taglib

Taglib segfaults - Null String