return an error when the server list does not contain the address

This is to avoid the error:

2023-09-25 14:43:23 Allocating openstack:cirros-64...
panic: runtime error: index out of range [0] with length 0

goroutine 30 [running]:
github.com/snapcore/spread/spread.(*openstackProvider).waitServerCompleteSetup(0xc0000ae240,
0xc000100e60, 0xf0, 0x0, 0x0)

/home/test/src/github.com/snapcore/spread/spread/openstack.go:379 +0x5bc
github.com/snapcore/spread/spread.(*openstackProvider).createMachine(0xc0000ae240,
0x9183c0, 0xc000069240, 0xc0000a8b40, 0x0, 0xc0000e39d8, 0xc0000a90e0)

/home/test/src/github.com/snapcore/spread/spread/openstack.go:511 +0xbbb
github.com/snapcore/spread/spread.(*openstackProvider).Allocate(0xc0000ae240,
0x9183c0, 0xc000069240, 0xc0000a8b40, 0xc0000a9168, 0xc0000489c8,
0x41a4e3, 0x8)

/home/test/src/github.com/snapcore/spread/spread/openstack.go:143 +0x6e
github.com/snapcore/spread/spread.(*Runner).allocateServer(0xc0000ac900,
0xc00006f080, 0xc0000a8b40, 0x0)
	/home/test/src/github.com/snapcore/spread/spread/runner.go:887
+0x25a
github.com/snapcore/spread/spread.(*Runner).client(0xc0000ac900,
0xc00006f080, 0xc0000a8b40, 0x0)
	/home/test/src/github.com/snapcore/spread/spread/runner.go:769
+0x767
github.com/snapcore/spread/spread.(*Runner).worker(0xc0000ac900,
0xc00006f080, 0xc0000a8b40, 0xc00014c248, 0x1, 0x1)
	/home/test/src/github.com/snapcore/spread/spread/runner.go:536
+0xa0
created by github.com/snapcore/spread/spread.(*Runner).loop
	/home/test/src/github.com/snapcore/spread/spread/runner.go:234
+0x702

spread/openstack.go

@@ -376,7 +376,11 @@ func (p *openstackProvider) waitServerCompleteSetup(s *openstackServer, timeoutS
 	// The adreesses for a network is map of networks and list of ip adresses
 	// We are configuring just 1 network address for the network
 	// To get the address is used the first network
-	s.address = server.Addresses[s.d.Networks[0]][0].Address
+	if len(server.Addresses) > 0 && len(s.d.Networks) > 0 {
+		s.address = server.Addresses[s.d.Networks[0]][0].Address
+	} else {
+		return fmt.Errorf("cannot retrieve server address")
+	}
 
 	config := &ssh.ClientConfig{
 		User:            "root",