dhcp: fix startup warnings with dhcp=internal

If asked to read a file that doesn't exist, sd_dhcp_lease_load()
returns 0 (success) without setting the out lease argument. So we need
to check both the return status and the lease before proceeding.

src/dhcp-manager/nm-dhcp-systemd.c

@@ -390,7 +390,7 @@ nm_dhcp_systemd_get_lease_ip_configs (const char *iface,
 
 	path = get_leasefile_path (iface, uuid, FALSE);
 	r = sd_dhcp_lease_load (&lease, path);
-	if (r == 0) {
+	if (r == 0 && lease) {
 		ip4_config = lease_to_ip4_config (lease, NULL, 0, FALSE, NULL);
 		if (ip4_config)
 			leases = g_slist_append (leases, ip4_config);