hostip: fix crash in sync resolver builds that use DOH

- Guard some Curl_async accesses with USE_CURL_ASYNC instead of
  !CURLRES_SYNCH.

This is another follow-up to 8335c64 which moved the async struct from
the connectdata struct into the Curl_easy struct. A previous follow-up
6cd167a fixed building for sync resolver by guarding some async struct
accesses with !CURLRES_SYNCH. The problem is since DOH (DNS-over-HTTPS)
is available as an asynchronous secondary resolver the async struct may
be used even when libcurl is built for the sync resolver. That means
that CURLRES_SYNCH and USE_CURL_ASYNC may be defined at the same time.

Closes https://github.com/curl/curl/pull/6603

0 files changed, 0 insertions, 0 deletions