MFC r309563: pflog: Correctly initialise subrulenr

subrulenr is considered unset if it's set to -1, not if it's set to 1.
See contrib/tcpdump/print-pflog.c pflog_print() for a user.

This caused incorrect pflog output (tcpdump -n -e -ttt -i pflog0):

rule 0..16777216(match)

instead of the correct output of

rule 0/0(match)

PR: 214832
Submitted by: andywhite@gmail.com