pf: ensure that we won't enter an endless loop

ensure that we won't enter an endless loop while iterating over
an address pool. problem found and solution tested by claudio.
ok claudio, henning, "reads fine" to zinke

Obtained from: OpenBSD, mikeb <mikeb@openbsd.org>, e4fc4bddb9
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46927