timers: don't close interval timers when unrefd

This change fixes a regression introduced by commit
0d051238, which contained a typo that
would cause every unrefd interval to fire only once.

Fixes: https://github.com/joyent/node/issues/8900


Reviewed-By: Timothy J Fontaine <tjfontaine@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-by: Trevor Norris <trev.norris@gmail.com>