select: don't always assume pending I/O

This fixes a bug where timers would not be properly invoked if no I/O
was pending

Change-Id: If7e79d3337bcc3a61ac6cae3920d8e9cb15a200c
Reviewed-on: http://review.couchbase.org/32926
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>