Fix issue #166: Null reference error

Author: lvjp

lib/checks.js

@@ -191,10 +191,10 @@ var commands = module.exports = {
     var cmd = /\ [^-].*/.exec(args);
     if (cmd != null) {
       var options = args.substring(0, cmd.index).match(/--\w+/g);
-      if (!options.reduce((valid, item) => valid &&
-        (allowedoptions.indexOf(item) > -1), true)) return ['invalid_format'];
-
       if (options != null) {
+        if (!options.reduce((valid, item) => valid &&
+          (allowedoptions.indexOf(item) > -1), true)) return ['invalid_format'];
+
         var optparams = args.substring(0, cmd.index).match(/--\w+=\d+\w*/g);
         return ((optparams != null) &&  (optparams.length == options.length)) ?
           [] : ['healthcheck_options_missing_args'];