Update evdns.c

In function evdns_getaddrinfo
API evdns_base_resolve_ipv6 may return null at line no 4682
afterwards we are trying to dereference it below
if (want_cname)
{
data->ipv6_request.r->current_req->put_cname_in_ptr = &data->cname_result;
}
which may cause problem
To resolve checking the data->ipv6_request.r before dereferencing it as below
if (want_cname ) 
is replaced with
if (want_cname && data->ipv6_request.r)