fixed showing no 'Usage'.

tools/mruby/mruby.c

@@ -60,6 +60,8 @@ parse_args(mrb_state *mrb, int argc, char **argv, struct _args *args)
 
   memset(args, 0, sizeof(*args));
 
+  if (argc == 1) return -2;
+
   for (argc--,argv++; argc > 0; argc--,argv++) {
     char *item;
     if (argv[0][0] != '-') break;
@@ -121,7 +123,7 @@ append_cmdline:
       else return -3;
       return 0;
     default:
-      break;
+      return -4;
     }
   }