support: Add check for TID zero in support_wait_for_thread_exit

Some kernel versions (observed with kernel 5.14 and earlier) can list
"0" entries in /proc/self/task.  This happens when a thread exits
while the task list is being constructed.  Treat this entry as not
present, like the proposed kernel patch does:

[PATCH] procfs: Do not list TID 0 in /proc/<pid>/task
<https://lore.kernel.org/all/8735pn5dx7.fsf@oldenburg.str.redhat.com/

>

Fixes commit 032d74ea ("support: Add
support_wait_for_thread_exit").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit 176c88f5)