LIB-48: NULL/-1 on not success

This basically reverts 4700fbe8f12452f7179e4c5544cbdb8b90c2fd5e, but keeps some style changes it introduced and also reverts e2991487504b9e45968dfb8c98b1a9af5595aca5.

When no read/put was successfull we return NULL for strings and -1 for numbers.
Additionally we issue an assert, like we did before the changes.

However, we now assert disc->success on everything that would get data from the structure. Some functions missed that before.

still tracked in http://tickets.musicbrainz.org/browse/LIB-48