Minor logic fix

music_directory     "${cfg.musicDirectory}"
playlist_directory  "${cfg.dataDir}/playlists"
db_file             "${cfg.dataDir}/tag_cache"
state_file          "${cfg.dataDir}/state"
sticker_file        "${cfg.dataDir}/sticker.sql"

all refer to ${cfg.datadir}, which by default is "/var/lib/mpd/". 

Parsing for instance ${cfg.datadir}/playlists results in:
/var/lib/mpd//playlists - NOT FOUND.
Merge pull request #2990 from third3ye/patch-3

Minor logic fix