Fix pfdenied not returning any results

When _a is empty we end up with an invalid invocation of pfctl, and no output.
We must add quotes to make it clear to pfctl that we're passing an empty anchor
name.

PR:		224415
Submitted by:	sigsys AT gmail.com
MFC after:	2 weeks

usr.sbin/periodic/etc/security/520.pfdenied

@@ -46,7 +46,7 @@ then
 	TMP=`mktemp -t security`
 	for _a in "" $(pfctl -a "blacklistd" -sA 2>/dev/null)
 	do
-		pfctl -a ${_a} -sr -v -z 2>/dev/null | \
+		pfctl -a "${_a}" -sr -v -z 2>/dev/null | \
 		nawk '{if (/^block/) {buf=$0; getline; gsub(" +"," ",$0); if ($5 > 0) print buf$0;} }' >> ${TMP}
 	done
 	if [ -s ${TMP} ]; then