Crashes when saving traces when mcount is never called.

[tlambertz]

Creating fake uftrace data dir at /myfs..
  Creating ./info
    feats = TASK_SESSION
    info = CMDLINE | TASKINFO
    cmdline = 'fakeuftrace'
    tid = []
thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', src/frontend.rs:131:38

[tlambertz]

Fixed in d86e3f6