c-ares: fix cancelled resolves

When built IPv6-enabled, we could do Curl_done() with one of the two
resolves having returned already, so when ares_cancel() is called the
resolve callback ends up doing funny things (sometimes resulting in a
segfault) since it would try to actually store the previous resolve even
though we're shutting down the resolve.

This bug was introduced in commit 8ab137b2 so it hasn't been
included in any public release.

Bug: http://curl.haxx.se/bug/view.cgi?id=3145445
Reported by: Pedro Larroy