- curl_multi_timeout() could return a timeout value of 0 even though nothing

- curl_multi_timeout() could return a timeout value of 0 even though nothing
  was actually ready to get done, as the internal time resolution is higher
  than the returned millisecond timer. Therefore it could cause applications
  running on fast processors to do short bursts of busy-loops.
  curl_multi_timeout() will now only return 0 if the timeout is actually
  alreay triggered.