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
@oschaaf right, this makes native fetching always fail. I'm not familiar enough to know if it is possible to have ipv6 results in the older Tengine versions (if compiled with `--with-ipv6`). On second look it seems like it is not possible, so I can switch it to just take the first result and then squash the commits if that sounds good to you.