curl_multi_wait: only use internal timer if not -1

commit 29bf0598 introduced a problem when the "internal" timeout is
prefered to the given if shorter, as it didn't consider the case where
-1 was returned. Now the internal timeout is only considered if not -1.

Reported-by: Tor Arntsen
Bug: http://curl.haxx.se/mail/lib-2013-06/0015.html