fix for mxcheck when domain has MX but has not A record (via Net::DNS)

this bug was introduced in Email-Valid-1.193

to demonstrate install `Net::DNS` and try:

```
perl -MEmail::Valid -E "say Email::Valid->address( -address=>'test@asqtstdmn.auditsquare.com', -mxcheck=>1) ? 'yes' : 'no'"
```