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().

Author: adoy

ext/curl/multi.c

@@ -191,6 +191,9 @@ PHP_FUNCTION(curl_multi_select)
 	FD_ZERO(&exceptfds);
 
 	curl_multi_fdset(mh->multi, &readfds, &writefds, &exceptfds, &maxfd);
+	if (maxfd == -1) {
+		RETURN_LONG(-1);
+	}
 	RETURN_LONG(select(maxfd + 1, &readfds, &writefds, &exceptfds, &to));
 }
 /* }}} */