[coverity] playlist.c get_track_filename() uninitialized buffer

format_track_path() uses path_append_ex() which might use strlen on the
existing buffer likely a false positive but cheap enough to guard against

Change-Id: I7061a434460f235fc58016f9dcf2dfe4633b9f59

apps/playlist.c

@@ -1042,6 +1042,8 @@ static int get_track_filename(struct playlist_info* playlist, int index,
     char tmp_buf[MAX_PATH+1];
     char dir_buf[MAX_PATH+1];
     bool utf8 = playlist->utf8;
+    if (buf_length > 0)
+        buf[0] = '\0';
 
     if (index < 0 || index >= playlist->amount)
         return -1;