Check mrb_open return value for NULL

tools/mruby/mruby.c

@@ -142,6 +142,11 @@ main(int argc, char **argv)
   struct _args args;
   struct mrb_parser_state *p;
 
+  if (mrb == NULL) {
+    fprintf(stderr, "Invalid mrb_state, exiting mruby");
+    return EXIT_FAILURE;
+  }
+
   n = parse_args(mrb, argc, argv, &args);
   if (n < 0 || (args.cmdline == NULL && args.rfp == NULL)) {
     cleanup(mrb, &args);