confmon test: don't memset listener

'listener' is not a POD and contains a std::set.

Change-Id: I9e474c21002f1614f7627a65f551440716f30163
Reviewed-on: http://review.couchbase.org/34144
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>