leaseshelper: fix crash when no mac is specified

If dnsmasq specified DNSMASQ_IAID (so we're dealing with an IPv6
lease) but no DNSMASQ_MAC, we skip creation of the new lease object.

Also skip adding it to the leases array.

https://bugzilla.redhat.com/show_bug.cgi?id=1202350

src/network/leaseshelper.c

@@ -439,7 +439,7 @@ main(int argc, char **argv)
 
     case VIR_LEASE_ACTION_OLD:
     case VIR_LEASE_ACTION_ADD:
-        if (virJSONValueArrayAppend(leases_array_new, lease_new) < 0) {
+        if (lease_new && virJSONValueArrayAppend(leases_array_new, lease_new) < 0) {
             virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                            _("failed to create json"));
             goto cleanup;