Fix segfault when enabling pf plug-ins

This fixes an issue where a segfault happens in pf_cn_test() if no
packet filtering rules have been parsed.  See the trac ticket for
more details.

Trac: 163
Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: David Sommerseth <davids@redhat.com>