When called on an unrealized chardev (e.g. from
qmp_qom_list_properties), s->addr is NULL. Return an error instead of
assert().
Fixes: 123676e989 ("char-socket: add 'addr' property")
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>