macosx: Unify fetching in VLCLibraryDataTypes.m

This helps curb duplicated code in several classes (i.e. the audio group types such as albums, artists, etc.)

This PR also adds checks of the pointers returned by the media library item fetching functions (there are real conditions in which a null pointer could be returned by these, causing a crash)

Signed-off-by: Claudio Cambra claudio.cambra@gmail.com