Check type of resolver_ctx->addrs.

Check the type of resolver_ctx->addrs and make sure that it is
ngx_addr_t* instead of in_addr_t*. addresses issue #839
@crowell This makes native fetching always fail for older Tengine versions, right?
I suspect it will not be possible to get ipv6 results here in that case, perhaps we can take the first result here instead to allow the native fetcher to work?