BUG/MINOR: checks: Don't subscribe to I/O events if it is already done

Subscription to I/O events should not be performed if the check is already
subscribed.

No backport needed.

(cherry picked from commit 9e0b3e92f73b6715fb2814e3d09b8ba62270b417)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed