Check an arguments count before proceed in sysctl_handler().

PR:		bin/56298
Submitted by:	Kang Liu <liukang@bjpu.edu.cn>
MFC after:	2 weeks

# We need a regression test suit for ipfw(2)/ipfw(8) badly.

sbin/ipfw/ipfw2.c

@@ -1643,7 +1643,7 @@ sysctl_handler(int ac, char *av[], int which)
 	ac--;
 	av++;
 
-	if (*av == NULL) {
+	if (ac == 0) {
 		warnx("missing keyword to enable/disable\n");
 	} else if (strncmp(*av, "firewall", strlen(*av)) == 0) {
 		sysctlbyname("net.inet.ip.fw.enable", NULL, 0,