ares::Curl_resolver_cancel: skip checking for NULL conn

Coverity CID 1243581. 'conn' will never be NULL here, and if it would be
the subsequent statement would dereference it!
multi_runsingle: fix possible memory leak

Coverity CID 1202837. 'newurl' can in fact be allocated even when
Curl_retry_request() returns failure so free it if need be.