libexec/rc.d/hostapd: Down/up interface when interface is specified

When no interface is specified results in a syntax error in the rc
script. Only execute poststart when an interface has been specified.

PR:		263358
Submitted by:	markj
Reported by:	Joshua Kinard <freebsd@kumba.dev>
Fixes:		0da2c91e64
MFC after:	3 days

libexec/rc/rc.d/hostapd

@@ -15,9 +15,11 @@ command=${hostapd_program}
 start_postcmd="hostapd_poststart"
 
 hostapd_poststart() {
-	ifconfig ${ifn} down
-	sleep 2
-	ifconfig ${ifn} up
+	if [ -n "$ifn" ]; then
+		ifconfig ${ifn} down
+		sleep 2
+		ifconfig ${ifn} up
+	fi
 }
 
 ifn="$2"