Fix album artist category (#330)
Fix album artist category (#330)

Was broken at least on NC13 and postgres

Error was:
```
Doctrine\DBAL\Exception\TableNotFoundException: An exception occurred while executing 'SELECT DISTINCT(AB."artist_id") AS "id", AA."name", LOWER(AA."name") AS "lower" FROM "oc_audioplayer_albums" "AB" JOIN "oc_audioplayer_artists" AA ON AB."artist_id" = AA."id" WHERE AB."user_id" = ? ORDER BY LOWER(AA."name") ASC ' with params ["username"]: SQLSTATE[42P01]: Undefined table: 7 ERROR: missing FROM-clause entry for table "ab" LINE 4: ON AB."artist_id" = AA."id"
```

I did not test other setups.
Rello/audioplayer,Rello/audioplayer