sockets: avoid crash when cleaning up sockets for an invalid FD

If socket_listen_cleanup is passed an invalid FD, then querying the socket
local address will fail. We must thus be prepared for the returned addr to
be NULL

Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
