lisa.trace: Shield against trace-cmd command name reporting issue

trace-cmd sometimes does not show the command name when raw-parsing the
sched_switch and sched_wakeup events. Instead, it reports "<...>".

Make that an illegal value for TaskID.comm, so that we catch the issue as early
as possible, without going through every dataframe created by df_events(). Note
that the issue can appear for other events as well, although it's expected that
it will always appear for sched_switch/sched_wakeup. Checking at this level
should therefore be enough to catch traces with that issue.

trace-cmd bug report: https://bugzilla.kernel.org/show_bug.cgi?id=204979