The thread record keeps the Worker's name as an immortal string and frees
it with the record. worker.threadName and the in-thread threadName
returned that buffer as a string value, whose retains do not extend an
immortal string's lifetime, so a name read before the Worker was released
pointed at freed memory once the record was destroyed. Both getters now
copy the name into a new string.
Corpus 5992 keeps worker.threadName past the worker's exit and the
Worker's release and then uses it; it matches Node, plain and with
SCRIPTC_SAN=1, and fails under ASan without this change.
Chris Tate
4530547f01