hostip: fix crash in sync resolver builds that use DOH #6603
This is another follow-up to 8335c64 which moved the async struct from
IMO the situation is confusing and it's easy to forget the difference. It could happen again.
caught during doh testing in a --disable-threaded-resolver build
- 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 asyncronous 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 #xxxx