song: get rid of newNullSong()

It didn't save us any lines of code nor did it do anything
useful since we would overwrite everything anyways.

src/song_save.c

@@ -111,7 +111,7 @@ void readSongInfoIntoList(FILE *fp, struct songvec *sv,
 			if (song)
 				insertSongIntoList(sv, song);
 
-			song = newNullSong();
+			song = xmalloc(sizeof(*song));
 			song->url = xstrdup(buffer + strlen(SONG_KEY));
 			song->type = SONG_TYPE_FILE;
 			song->parentDir = parentDir;