Prevent reading from a nil response in DNS64

[EkilDeew]

Sometimes if the upstream failed to be contacted, the return response will be empty.
DNSCrypts still wants to read the response code. This causes a panic if DNS64 is configured.

[EugeneOne1]

@EkilDeew, hello and thanks for the contribution.

The fix is rather simple, but I presume there is a better place for the introduced check. The documentation of checkDNS64 function states both the request and response are required to be non-nil, so I propose to move the check to the only place this function called from – the performDNS64 function, right above the call itself. Perhaps, it even makes sense to check the request for nilness as well.

[EugeneOne1]

See #323 (comment).

[EkilDeew]

I've added the check in performDNS64.
However I've only added the response check, because this was the nil object.
The request object should always be non-nil, and if not should crash in developpement because if would likely be a dnsproxy error rather than an third party induced error (as the error in this PR was).

[EugeneOne1]

@EkilDeew, it's all right now, we've merged that. Thanks again for the contribution.