threaded resolver: fix timeout issue

Reset old timer first so we can set a new one further in the future.

lib/hostthre.c

@@ -451,6 +451,9 @@ CURLcode Curl_is_resolved(struct connectdata *conn,
 
     td->interval_end = elapsed + td->poll_interval;
 
+    /* Reset old timer so we can set a new one further in the future */
+    Curl_expire(conn->data, 0);
+
     Curl_expire(conn->data, td->poll_interval);
   }