song: disallow operations on MPD_TAG_FILE
song: disallow operations on MPD_TAG_FILE

MPD_TAG_FILE is initialized with mpd_song_new() and should not be
modified later.

jwise/libmpdclient,OpenInkpot-archive/iplinux-libmpdclient,yusiwen/libmpdclient,OpenInkpot-archive/iplinux-libmpdclient,sl0th/libmpdclient,jwise/libmpdclient,sl0th/libmpdclient,jwise/libmpdclient,jagleeso/libmpdclient,yusiwen/libmpdclient,yusiwen/libmpdclient,sl0th/libmpdclient,psych0tik/libmpdclient2,cmende/libmpdclient,psych0tik/libmpdclient2,cmende/libmpdclient,jagleeso/libmpdclient,jagleeso/libmpdclient,cmende/libmpdclient