dns: fix crash using dns.setServers after resolve4

The callback function in cares_query is synchronous and called before
closed. So dns.setServers in the synchronous callback before closed will
occur crashing.

Fixes: https://github.com/nodejs/node/issues/894
Refs: https://github.com/nodejs/node/blob/v6.9.4/deps/cares/src/ares_process.c#L1332-L1333
Refs: https://github.com/OpenSIPS/opensips/blob/2.3.0/proxy.c
PR-URL: https://github.com/nodejs/node/pull/13050


Reviewed-By: Anna Henningsen <anna@addaleax.net>