libutils: Don't segfault ProcessCallStack when getting thread name fails

There's an inherent race in trying to read out the thread name from
the system and the thread closing out on its own (and thus being removed
from procfs).

Try to handle this by formatting the thread name unconditionally with
the tid when getting the thread name fails (instead of dereferencing
NULL and crashing).

Bug: 15406837
Change-Id: Ibf2208b8ce345589e7e9c57e6a307728d1121c5d