Similarly to -thread-info, -thread-list-ids, -list-thread-groups (--available)
should always be issuable without a live selected thread.

I tested it in non-stop mode, and all-stop sync/async as well with
success, with an exited thread selected.

In the all-stop async case, '-list-thread-groups' always works,
even if the target is running, because it isn't (currently) asking
the remote side for anything.  OTOH, '-list-thread-groups --available' will
error out with "Cannot execute this command while the target is running.",
which is expected due to limitations of the all-stop remote protocol
(same error as given by -thread-info).  In non-stop mode, it always
works.

Checked in.

-- 
Pedro Alves