patch attempt to reference undefined optarg (credit Paul Farrall)

git-svn-id: https://nagiosplug.svn.sourceforge.net/svnroot/nagiosplug/nagiosplug/trunk@612 f882894a-f735-0410-b71e-b25c423dba1c

plugins/check_http.c

@@ -348,7 +348,7 @@ process_arguments (int argc, char **argv)
 
 		switch (c) {
 		case '?': /* usage */
-			usage2 ("unknown argument", optarg);
+			usage3 ("unknown argument", optopt);
 			break;
 		case 'h': /* help */
 			print_help ();