Issue #11393: Fix faulthandler_thread(): release cancel lock before join lock

If the thread releases the join lock before the cancel lock, the thread may
sometimes still be alive at cancel_dump_tracebacks_later() exit. So the cancel
lock may be destroyed while the thread is still alive, whereas the thread will
try to release the cancel lock, which just crash.

Another minor fix: the thread doesn't release the cancel lock if it didn't
acquire it.