Fix spamlogd's rc_pre to avoid creating pflog0 if spamd is disabled.

Fix spamlogd's rc_pre to avoid creating pflog0 if spamd is disabled.
The conditional was not checked (prior to r1.2 the exit code from the
conditional was implicitly used as the exit code from the whole shell
function, but this wasn't adapted when pflog creation was added).
Reported/tested by Clint Pachl