core: Fix FencingPolicy copy constructor

Properly initializes hostsWithBrokenConnectivityThreshold when instance
is created using copy constructor with null parameter.

Change-Id: I3b1445e404887704562d9d8bbfdf733b75924f22
Bug-Url: https://bugzilla.redhat.com/1120829
Signed-off-by: Martin Perina <mperina@redhat.com>