Unfortunately, I wasn't able to reproduce the issue and neither the
reporter can reproduce it anymore and we can't known exactly why we get
to situation when uri and sockaddr is NULL without the logs.
service->uri and service->sockaddr are assigned through failover
callback, see ad_resolve_callback for example, which is probably used in
the reporter's environment. My suspicion is that for some reason we
can't assign those attributes (which may happen for several reasons) but
since failover callback doesn't report any error failover actually
succeeds and we end up dereferencing NULL.
I don't think we can move any further without logs but this is at least
a hotfix that will prevent segfault.