hostip: fix crash in sync resolver builds that use DOH #6603
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!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
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