MINOR: tasks/debug: add some extra controls of use-after-free in DEBUG_TASK

MINOR: tasks/debug: add some extra controls of use-after-free in DEBUG_TASK

It's pretty easy to pre-initialize the index, change it on free() and check
it during the wakeup, so let's do this to ease detection of any accidental
task_wakeup() after a task_free() or tasklet_wakeup() after a tasklet_free().
If this would ever happen we'd then get a backtrace and a core now. The
index's parity is respected so that the call history remains exploitable.