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!

lib/asyn-ares.c

@@ -178,7 +178,7 @@ static void destroy_async_data (struct Curl_async *async);
  */
 void Curl_resolver_cancel(struct connectdata *conn)
 {
-  if(conn && conn->data && conn->data->state.resolver)
+  if(conn->data && conn->data->state.resolver)
     ares_cancel((ares_channel)conn->data->state.resolver);
   destroy_async_data(&conn->async);
 }