Avoid calling select if maxfd returned by curl_multi_fdset is -1

Avoid calling select if maxfd returned by curl_multi_fdset is -1

As per libcurl documentation :

When libcurl returns -1 in max_fd, it is because libcurl currently
does something that isn't possible for your application to monitor
with a socket and unfortunately you can then not know exactly when
the current action is completed using select().